[Repo Assist] improve: use SearchValues(char) in ShellQuoting.NeedsQuoting for SIMD metachar scan

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Summary

Replaces the per-character foreach + IsShellMetachar switch in ShellQuoting.NeedsQuoting with System.Buffers.SearchValues<char>, which builds a vectorized lookup structure at startup and uses IndexOfAny to scan for metacharacters in a single SIMD pass.

Root cause / motivation

NeedsQuoting is called on every argument in every system.run / system.run.prepare invocation. The former implementation iterated character-by-character through a 25-arm switch expression. On .NET 8+, SearchValues<char> uses SSE2/AVX2 instructions to check multiple characters against a lookup bitmap simultaneously — significantly faster for typical argument lengths.

Changes

File	Change
ShellQuoting.cs	Add using System.Buffers; add static readonly SearchValues<char> s_shellMetachars (same 25-char set as IsShellMetachar); replace NeedsQuoting body with arg.AsSpan().IndexOfAny(s_shellMetachars) >= 0; remove now-unused IsShellMetachar private helper

Net diff: 9 insertions / 16 deletions — net −7 lines while preserving identical semantics.

Metacharacter set preserved (25 chars)

space, \t, ", ', &, |, ;, <, >, (, ), ^, %, !, $, `, *, ?, [, ], {, }, ~, \n, \r

Identical to the former IsShellMetachar switch — no behavioral change.

Trade-offs

• SearchValues<char> requires System.Buffers (using added) — available since .NET 8; this project targets net10.0.
• The static field is initialized once; subsequent calls pay only the span scan cost.
• No change to QuoteForShell or FormatExecCommand — only the metachar detection inner loop is affected.

Test Status

All existing ShellQuotingTests pass unchanged (covering all 25 metacharacters, empty/plain args, QuoteForShell cmd and PowerShell paths, FormatExecCommand edge cases).

Shared.Tests: 586 passed, 20 skipped — exit 0
Tray.Tests: 122 passed — exit 0

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