Refactor a few internal regex-based helpers

[matthewp]

Changes

• Replaces extension and base-path regex helpers with direct string handling in content and preview utilities.
• Simplifies content render transform filtering so it no longer needs per-entry dynamic regex generation.

Testing

• Installed dependencies in a fresh worktree to validate the split branch in isolation.
• Ran Biome against the touched source files and changeset to confirm the branch stays formatted and lint-clean.

Docs

• No docs update needed, because this only changes internal implementation details.

[changeset-bot]

🦋 Changeset detected

Latest commit: 61f56bb

The changes in this PR will be included in the next version bump.

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[matthewp]

Moving this to draft, I don't know if all of these are necessary.

[codspeed-hq]

Merging this PR will not alter performance

✅ 18 untouched benchmarks

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_{Comparing semgrep-regex-refactors (61f56bb) with main (a002540)¹}

Footnotes

1. No successful run was found on main (32b361d) during the generation of this report, so a002540 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[matthewp]

I don't think these are necessary, closing.