Replace upgrade.sh with Go-based upgrade tool in tools/upgrade/

.gitignore

@@ -23,3 +23,5 @@ go.work
 # Upgrade script work/report directories
 .upgrade-work/
 .upgrade-report/
+
+./tools/upgrade/upgrade

tools/upgrade/README.md

@@ -0,0 +1,76 @@
+# upgrade
+
+Automates the TinyGo `net` package upgrade process by backporting
+changes from upstream Go stdlib and generating comparison reports.
+
+It handles three categories of files:
+
+- **TinyGo-only** (e.g. `netdev.go`, `tlssock.go`) — skipped entirely
+- **Unmodified copies** — replaced directly from upstream
+- **Modified files** (with `// TINYGO` markers) — 3-way merged via `diff3`
+
+## Prerequisites
+
+- `diff` and `diff3` (from GNU diffutils)
+- Go source trees are resolved automatically:
+  1. gvm installs at `~/.gvm/gos/`
+  2. Active `GOROOT` if the version matches
+  3. Downloaded from `https://go.dev/dl/` into `.upgrade-work/`
+
+## Build
+
+```sh
+cd tools/upgrade
+go build -o upgrade .
+```
+
+This produces the `upgrade` binary. It must be run from the `net`
+package root directory (the repository root).
+
+## Usage
+
+```sh
+# From the net package root:
+./tools/upgrade/upgrade [flags]
+```
+
+### Flags
+
+| Flag | Default | Description |
+|------|---------|-------------|
+| `--dry-run` | `false` | Preview changes without modifying files |
+| `--cur` | `1.21.4` | Go version the TinyGo net package is currently based on |
+| `--upstream` | `1.26.2` | Target upstream Go version to upgrade to |
+| `--file` | | Process a single file instead of all files |
+
+### Examples
+
+```sh
+# Preview what would change
+./tools/upgrade/upgrade --dry-run
+
+# Perform the upgrade
+./tools/upgrade/upgrade --cur 1.21.4 --upstream 1.26.2
+
+# Preview a single file
+./tools/upgrade/upgrade --dry-run --file dial.go
+```
+
+## Output
+
+Reports are written to `.upgrade-report/` in the net package root:
+
+- `summary.txt` — one-line status per file
+- `diffs/` — unified diffs showing upstream and TinyGo changes
+- `merged/` — merge results (only in apply mode)
+
+Files with merge conflicts are saved as `<file>.conflicted` alongside
+the original.
+
+## After upgrading
+
+1. Review and resolve any `.conflicted` files
+2. Check for files missing from upstream (may have been renamed/removed)
+3. Verify `// TINYGO` comments are preserved
+4. Test with TinyGo example/net examples
+5. Update `README.md` version references

tools/upgrade/go.mod

@@ -0,0 +1,3 @@
+module github.com/tinygo-org/net/tools/upgrade
+
+go 1.22