Hooks: Allow per-OS defines

[palmoni5]

#406 made source and name accept a map with per-OS entries and a default. This PR extends
the same syntax to defines, so one source: source build can use different compile-time options
per target. For example, SQLITE_WIN32_MALLOC only on Windows, or a smaller set of options on
mobile:

hooks:
  user_defines:
    sqlite3:
      source: source
      path: path/to/sqlite3.c
      defines:
        windows:
          - SQLITE_WIN32_MALLOC
        default:
          default_options: false
          defines:
            - SQLITE_THREADSAFE=1

Behavior:

• Each entry accepts the same formats as defines today: a list, or a map with default_options
  and defines.
• A map counts as per-OS only when its keys are OS names (OS.name, as for source) or
  default. The existing keys (default_options, defines) are neither, so existing
  configurations keep their meaning (there's a test for that).
• A map that mixes both kinds of keys throws an ArgumentError instead of being silently
  misread.
• If the target OS has no entry and there's no default, the default options are used, the same
  as when defines is missing.

I only did this for defines. path, additional_flags and the other source: source options
(including additional_sources from #395) could get the same treatment, but I wanted to check with
you before widening the change.

Tests

• New tests in test/hook/description_test.dart cover the entry for the target OS, the default
  fallback, a missing entry, a plain options map, and mixed keys.
• End to end on Windows: source: source with SQLite 3.53.4 and
  defines: {windows: {defines: [SQLITE_ENABLE_RBU]}, default: [...]} builds a library where
  usedCompileOption('ENABLE_RBU') and usedCompileOption('ENABLE_FTS5') are both true, so the
  Windows entry is applied on top of the default options.
• dart analyze --fatal-infos and dart format --set-exit-if-changed in sqlite3: clean.
• dart test in sqlite3 (VM, Windows): 196 passed, 5 skipped.

🤖 Generated with Claude Code

[palmoni5]

Thanks! Done in a25e5ba: I removed the separate example and paragraph, and the existing paragraph now says the map syntax is available for source and defines.