docs: update architecture, decisions, and HyperCard mapping

[jrepp]

Summary

• Rebased docs/architecture-update onto current origin/main (f626f4f).
• Dropped the duplicate asset-browser .document switch commit during rebase because it is already on origin/main via PR fix(asset-browser): handle .document case in AssetKind switches #283.
• Leaves one docs/build commit: dc21e2d docs: update architecture, decisions, and HyperCard mapping.

What Changed

• Updates architecture.md and decisions.md around imported HyperCard project behavior, stack-library metadata, used-stack pass-up, external emulator outcomes, runtime navigation, and known compatibility gaps.
• Preserves current durable-doc frontmatter and current mainline design guardrails for target validation, AI context policy, SpriteKit authoring, visual QA, route-only import translation, and debug import isolation.
• Keeps architecture property/API tables aligned with source for selectedTime, slider tracking behavior, classic media lookup, chart/spider properties, and AI Context Library policy.
• Updates docs/HyperCardExternalMapping.md with durable compatibility-slice labels for PICT/palette/QuickDraw/HyperTint/window emulation instead of PR-series labels.
• Adds stackLibrary/legacyImport persistence notes to docs/SQLiteStackStorageDesign.md as non-breaking optional document_values metadata.
• Adds an imported UTI declaration for classic HyperCard stack files to script/build_and_run.sh, with a note that extensionless stack acceptance still depends on the importer’s .data fallback and stack-format validation path.

Actionable Review Focus

• Confirm the used-stack pass-up wording matches the intended message hierarchy: part/scene -> card/background -> active stack -> used stack scripts -> app.
• Check that the stackLibrary persistence wording matches current storage behavior: optional metadata in document_values, default-on-missing decode, and no document-version bump.
• Review whether the generated app plist should add any additional classic HyperCard UTI tags later; this PR currently only names the imported UTI and documents the importer fallback.

Verification

• Rebase completed locally and pushed with force-with-lease to origin/docs/architecture-update.
• git diff --check origin/main...HEAD passed.
• Scanned for conflict markers, stale PR 7x labels, stale updateSpriteViews()/draw() wording, and narrowed chart/calendar property-table regressions.
• No Swift test suite was run; this PR is docs plus build-run plist metadata.