refactor(epub): Japanese CJK line-breaking PR D — ITextMetrics scaffolding + host characterization tests

[mtskf]

Japanese / CJK Line-Breaking — PR D (Phase 1: Testability Scaffolding)

First PR of the Japanese CJK line-breaking effort. Behavior-preserving refactor: it makes the text-layout code host-testable and pins the current layout behavior with characterization tests. No layout behavior changes (English or Japanese).

Why

The eventual fix (later PRs) generalises the word-join model so CJK paragraphs wrap at the page edge. Before touching layout, this PR extracts a thin text-metrics interface so ParsedText becomes host-testable, then locks down today's English / nbsp / hyphenation / BiDi output so any future regression is caught on the host.

Commits

1. refactor: extract ITextMetrics and detach TextBlock.h from HAL for host testing
   • New lib/GfxRenderer/ITextMetrics.h: pure-virtual interface with the 7 text-measurement methods ParsedText needs (native-safe — depends only on EpdFontFamily.h).
   • GfxRenderer now implements ITextMetrics (implicit up-cast; identical vtable dispatch — no behavior change).
   • TextBlock.h detached from the HAL: drop #include <HalStorage.h>, forward-declare class HalFile;, move the include into TextBlock.cpp. Adds read-only getWordStyles() / getWordXpos() accessors.
   • ParsedText.{h,cpp} now depend on const ITextMetrics& instead of const GfxRenderer&.
2. test: host harness pinning current ParsedText layout (English, nbsp, hyphenation, BiDi)
   • test/support/FakeTextMetrics.h: deterministic fixed-width ITextMetrics for host tests.
   • test/parsed_text/ParsedTextLayoutTest.cpp: 4 characterization tests pinning current break behavior (English word-wrap, no-break-space glue, Liang hyphenation split, BiDi RTL reorder).
   • CI sentinel gate (.github/workflows/ci.yml) + pre-commit hook (.githooks/pre-commit, appended — preserves the existing clang-format pass) guarding against vacuous placeholder tests.

Verification

• Host gtest suite green: 4/4 ParsedTextLayout tests pass, pristine under -Wall -Wextra -pedantic.
• Firmware build is verified by CI (no PlatformIO on the authoring host).
• Scope is exactly Phase 1; no src/ changes; no cache-format / SECTION_FILE_VERSION bump.

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