Support numbered output filenames with %n counters

[ekisu]

Summary

Support %n and zero-padded counters such as %04n in output_template. Select the first unused filename in the resolved output directory, including the current directory, and document gap/reset semantics.

Counters are captured before media-property expansion so counter-like text in a source filename stays literal. Conditional blocks that remove a counter do not trigger a filename search. Media substitutions use literal replacements for percent characters.

Fixes #167
Tracked in #217.

Verification

• During implementation, checked padded/unpadded counters, multiple counters, gap reuse, existing files, current-directory collisions, removed conditional counters and percent-containing filenames.
• Completed two real AVC encodes with clip_%04n: clip_0001.mp4 and clip_0002.mp4 (65,384 bytes each); the first file's SHA-256 remained unchanged.
• Build and CRLF-aware whitespace checks passed.
• Removed the standalone compiler-driven test at review request; those checks are not retained automated regression coverage in this PR.

Number selection occurs during formatting; concurrent independent encodes are not atomically reserved. No persistent counter is stored.

Current harness-based verification (supersedes historical test notes)

Rebased onto master 72d9d11, which includes #226 and #227. Tests load the normal built script and drive real mpv.

Three real-encode regressions cover padded sequential names, preserving existing files and filling gaps, and literal counter-looking source filenames. All three fail on the unfixed script. Pinned Nix: 17 tests, 16 passed, 1 existing audio skip.

Latest rebase verification

Rebased onto master 72d9d11 after #227 merged. Normal build and the complete pinned-Nix suite pass, with two expected skips (audio fixture and unavailable optional libaom-av1 encoder).

Review feedback addressed — 2026-09-12

Expanded output-template coverage to 10 focused real-mpv tests, including media placeholders, fractional timestamps, raw properties/fallback/conditions, escaped percent/counters and repeated padded counters. Full pinned-Nix suite: 37 tests, 35 passed and 2 expected skips. GitHub build/test checks pass.
Rebased onto master after #220, #221, #223 and #224 merged.

[ekisu]

Add other tests for the output template property

[ekisu]

Addressed in d87262b. Added real-encode coverage for %f/%F/%T, fractional start/end timestamps with and without milliseconds, raw property expansion, missing-property fallback, positive/negative conditions (both inclusion and omission), literal percent/escaped counters, mixed escaped percent plus real counters, and repeated differently padded counters. Tests assert expected filenames, decodable output and preserved existing bytes. The 10 focused template tests pass; the full pinned-Nix suite passes 35 tests with 2 expected skips.