feat(cli): add Kindle SDR highlight extraction

[AnnatarHe]

Summary

• add ck-cli sdr --path PATH [--json] with automatic Kindle tree, .sdr, and single-book discovery
• decode KRDS annotations and APNX pages, then reconstruct unencrypted AZW3/KF8 text natively in Go
• support uncompressed, PalmDOC, and HUFF/CDIC content with readable or ClippingItem JSON output
• document supported formats and add deterministic unit and end-to-end fixtures

Why

Kindle .azw3r sidecars store annotation positions rather than the highlighted words. Recovering the text requires joining those positions with the sibling book's reconstructed KF8 markup stream.

Impact

The command is read-only, offline, and has no Python or KindleUnpack runtime dependency. It extracts highlights, note spans, and underlines while preserving the existing JSON schema. DRM, Mobi7, and KFX/.yjr remain explicitly unsupported.

Validation

• go test ./...
• go vet ./...
• go build ./cmd/ck-cli
• CLI help and invalid-path smoke tests

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[claude]

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[codecov]

Codecov Report

❌ Patch coverage is 52.69017% with 510 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.68%. Comparing base (8518e5c) to head (3a0659c).

Files with missing lines	Patch %	Lines
internal/sdr/azw3.go	48.80%	202 Missing and 77 partials ⚠️
internal/sdr/krds.go	64.06%	60 Missing and 32 partials ⚠️
internal/sdr/extract.go	52.17%	53 Missing and 24 partials ⚠️
internal/commands/sdr.go	13.63%	31 Missing and 7 partials ⚠️
internal/sdr/text.go	67.60%	13 Missing and 10 partials ⚠️
cmd/ck-cli/main.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #16       +/-   ##
===========================================
+ Coverage   22.22%   43.68%   +21.45%     
===========================================
  Files           8       13        +5     
  Lines         594     1527      +933     
===========================================
+ Hits          132      667      +535     
- Misses        454      702      +248     
- Partials        8      158      +150     

Flag	Coverage Δ
unittests	43.68% <52.69%> (+21.45%)	⬆️

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