feat(seek): support independent seek offsets

[stackia]

Summary

• Support r2h-seek-offset=a,b so begin and end seek times can be shifted independently.
• Keep the existing single-value behavior by applying one offset to both begin and end times.
• Carry the separate offsets through HTTP/RTSP query append, URL templates, Range Seek begin-time handling, and configured service query merge.
• Update Chinese source docs and English translations with the new syntax.

Closes #516.

Validation

• cmake --build build -j$(getconf _NPROCESSORS_ONLN)
• ./scripts/run-e2e.sh test_url_template.py -k "seek_offset_pair or offset_pair"
• ./scripts/run-e2e.sh test_rtsp_seek_mode.py -k "offset_pair or configured_seek_offset_pair"
• ./scripts/run-e2e.sh test_url_template.py -k "seek_offset or offset"
• ./scripts/run-e2e.sh test_rtsp_seek_mode.py -k "offset"
• pnpm run lint:clang
• pnpm run lint:ruff
• pnpm run docs:build

[github-actions]

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[Copilot]

Pull request overview

This PR extends r2h-seek-offset to support an a,b form so begin/end seek times can be shifted independently, while preserving the legacy single-value behavior by applying one offset to both boundaries. The change is carried through seek parsing, HTTP/RTSP upstream query generation, URL templates, configured-service query merge behavior, and documentation.

Changes:

• Split seek offset handling into seek_begin_offset_seconds and seek_end_offset_seconds, and propagate through seek parsing + upstream conversion logic.
• Add/expand e2e coverage for offset pairs across URL templates, HTTP query append, RTSP query append, and Range Seek clock behavior.
• Update CN/EN docs to document r2h-seek-offset=a,b and clarify Range Seek semantics (begin-only for window/clock).

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/url_template.h	Updates seek_parse_result_t to store begin/end offsets separately.
src/stream.c	Passes per-boundary offsets into service_parse_seek_value() for HTTP/RTSP flows.
src/service.h	Updates service_t and seek-related APIs to use begin/end offsets.
src/service.c	Implements parsing of r2h-seek-offset as single or a,b, propagates offsets through parsing/conversion and config merge.
e2e/test_url_template.py	Adds e2e tests validating offset-pair behavior for templates and query append (including Unix timestamps).
e2e/test_rtsp_seek_mode.py	Adds e2e tests validating Range Seek uses begin offset for Range: clock= and config merge behavior for offset pairs.
docs/guide/url-formats.md	Documents offset-pair usage in CN URL examples and guidance.
docs/guide/time-processing.md	Documents a,b semantics and Range Seek begin-only behavior in CN.
docs/en/guide/url-formats.md	Documents offset-pair usage in EN URL examples and guidance.
docs/en/guide/time-processing.md	Documents a,b semantics and Range Seek begin-only behavior in EN.

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