fix(nap): heading-aware archival

[diberry]

Extends archiveDecisions() parser to recognize ## section headings as boundaries. Preserves section structure during archival — ## preambles are kept as atomic units tied to their ### children.

Changes:

• Entry parser now stops at both ## and ### headings (prevents ## headers from being swallowed into adjacent ### entry content)
• Tracks ## section boundaries and maps each ### entry to its parent section
• Reconstruction emits section preambles only when at least one child entry is present
• Budget calculation accounts for section preamble overhead
• 2 new tests: section heading preservation + orphan prevention

Working as EECOM (Core Dev)

Closes #22

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[diberry]

✅ Team Approved

Flight (Lead) and Coordinator reviewed and approved. 1 commit, 2 files (nap.ts + nap.test.ts), 44 tests pass (42 existing + 2 new). Parser correctly handles both ## and ### headings, preserves section structure atomically during archival. Ready to retarget upstream.

[diberry]

Retargeted to bradygaster#650