forms: add enctype + 5 submitter form-* IDL accessors

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What

Adds six missing form-submission IDL accessors so reads of form.enctype and submitter.{formAction, formEnctype, formMethod, formNoValidate, formTarget} return the spec-mandated string or boolean instead of undefined. Companion to #2279 (which wired up the content-attribute submission path for the same set of attributes) — same WHATWG sections, same canonicalization rules, just the JS-property side that downstream CDP clients read.

Also bundled: support for enctype="text/plain" form submissions per HTML §4.10.21.8. Before this PR the IDL accessor and the submission path silently agreed (both ignored text/plain). With the IDL accessor now returning "text/plain" per spec, the submission path was the next link in the chain — left untouched it would have re-encoded as urlencoded with a .not_implemented warning. Reviewers preferring to land just the read-only IDL accessors first can ask to drop commits 4b693db4..f0cce427; both are self-contained and revert cleanly.

Closes #2449.

Root cause

The accessors were absent from the JsApi blocks of Form.zig, Button.zig, and Input.zig. Reading form.enctype from JS resolved through V8 to "property not on the prototype" → undefined. Runtime.evaluate returned result.value === undefined. CDP clients that then call .toLowerCase() on the return crash with TypeError: Cannot read properties of undefined.

flowchart LR
    A[Runtime.evaluate form.enctype] --> B[no accessor on HTMLFormElement.prototype]
    B --> C[returns undefined, TypeError downstream]
    style B fill:#fdd
    style C fill:#fdd

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Fix

Commit 2fdc82aa — IDL accessors:

• src/browser/webapi/element/html/Form.zig — add getEnctype / setEnctype mirroring the existing getMethod / setMethod (limited-to-known-values pattern), register enctype in JsApi.
• src/browser/webapi/element/html/Button.zig — add five getter/setter pairs (getFormAction, getFormEnctype, getFormMethod, getFormNoValidate, getFormTarget) and register them in JsApi. URL-resolving getFormAction mirrors Form.getAction.
• src/browser/webapi/element/html/Input.zig — same five accessors registered alongside the existing form accessor.

Commit cedfdba0 — IDL DRY pass:

• Extract Form.normalizeMethod / Form.normalizeEnctype helpers; the five IDL callsites (existing Form.getMethod + four new accessors) collapse to one-liners. Net -36 LOC.

Commit 4b693db4 — text/plain submission:

• src/browser/webapi/net/FormData.zig — EncType gains a .plaintext variant; plaintextEncode writes name=value\r\n per entry per HTML §4.10.21.8.
• src/browser/Frame.zig — submitForm recognizes text/plain before the urlencoded fallback, sets Content-Type: text/plain; charset=<form-charset> per spec step 21.4.

Commit f0cce427 — submission-path DRY:

• src/browser/Frame.zig — submitForm now consumes Form.normalizeMethod / Form.normalizeEnctype instead of inlining the eqlIgnoreCase chain a sixth time. Single function owns the canonical mapping for both IDL reads and the submission path. Drops the log.warn(.not_implemented, "FormData.encoding", ...) branch — after text/plain lands, the only values still reaching the urlencoded fallback are spec-invalid ones, which per §4.10.21.5 silently canonicalize (Chrome doesn't log either).

Defaults follow WHATWG HTML strictly:

Accessor	Missing	Invalid
form.enctype	application/x-www-form-urlencoded	application/x-www-form-urlencoded
submitter.formEnctype	""	application/x-www-form-urlencoded
submitter.formMethod	""	get
submitter.formAction	document URL (frame.url)	resolved URL
submitter.formTarget	""	(no enumeration — passes through)
submitter.formNoValidate	false	true (boolean attribute reflection)

flowchart LR
    A[Runtime.evaluate form.enctype] --> B[Form.getEnctype reads attribute, normalizes]
    B --> C[returns application/x-www-form-urlencoded]
    style B fill:#dfd
    style C fill:#dfd

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Test

• HTML fixtures under src/browser/tests/element/html/:
  • form.html — three new <script> blocks covering enctype initial / set / normalization.
  • button.html — eleven new <script> blocks covering all five submitter overrides (initial / set / invalid-value / normalization).
  • input-attrs.html — eight new <script> blocks mirroring the button coverage on <input type=submit>.
• Zig unit tests under src/browser/webapi/net/FormData.zig:
  • FormData: plaintext write — three entries including one with embedded = and CRLF; asserts the spec's verbatim-write semantics.
  • FormData: plaintext empty body — empty entry list → empty output.
• All runners pass: TEST_FILTER='WebApi: HTML.Form', 'WebApi: HTML.Button', 'WebApi: HTML.Input', 'FormData: plaintext'. Full zig build test clean (639/639).
• Toggle-off check: reverting FormData.zig while keeping Frame.zig causes a compile failure on the new .plaintext variant (proves the production code routes through it); reverting all three webapi/element/html/*.zig production files while keeping the HTML fixtures causes the three runners to fail (proves the fixtures exercise the new accessors).
• Reproducer (Closes-linked issue forms: enctype + 5 submitter form-* IDL accessors return undefined #2449): a self-contained repro.sh exits 1 against installed nightly (1.0.0-nightly.6198+2e159aaf — all 11 IDL reads return undefined) and exits 0 against a local debug build off this branch (all 11 reads return spec-typed values, including formAction resolving to the document URL when missing).

Notes

• The submitter formAction getter intentionally diverges from the submitter.X || form.X short-circuit some downstream JS uses: per HTML §4.10.21.6 it MUST return the document URL when the attribute is missing, not "". Callers that want fall-through to the form's value should check submitter.hasAttribute("formaction") first (matches Chrome behavior).
• No CDP surface changes — this is purely a JS API addition. Existing CDP Network.requestWillBeSent payloads for submitter-triggered submissions are unaffected (those were addressed by forms: honor formaction / formmethod / formenctype on submit button #2279).