fix(search): ellipsize parity matrix labels by runes, not bytes

[dschmidt]

The byte-based shorten/shortenQuery in the parity matrix cut truncation points into the middle of multi-byte UTF-8 sequences. Visible on the search branch (#3211), whose fixtures include Motörhead: its committed parity README contains literally invalid UTF-8 (audio.arti...¶rhead=3, the ¶ being the orphaned second byte of ö).

This replaces both helpers with a shared rune-based ellipsize, so the cut counts characters and can never split a sequence. The matrix on main contains no affected labels and is unchanged; the broken README on the search branch heals itself with the next matrix regeneration there once this is merged.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

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0 new issues

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🟢 Metrics 0 complexity

Metric	Results
Complexity	0

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🟢 Coverage ∅ diff coverage · +0.00% coverage variation

Metric	Results
Coverage variation	✅ +0.00% coverage variation (-1.00%)
Diff coverage	✅ ∅ diff coverage

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Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (e848d46)	89089	21609	24.26%
Head commit (b2039f6)	89089 (+0)	21611 (+2)	24.26% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#3643)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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