fix: centralize regex sources and harden scan flows

[FanaticPythoner]

Regex construction routes runtime, tests, workflow checks, release
checks, perf harnesses, and evidence tooling through a typed registry
while preserving scan parity.

Regex registry:

• add shared fragments, named patterns, tag placeholders, default regex
  constants, builder helpers, and typed registry errors
• replace duplicated literals in detection, utils, attributes,
  highlights, icons, tree, stream scanning, config, webpack, perf,
  release, workflow, parity, and test code
• add an equivalence harness that covers 81/81 baseline sources,
  73/73 behavior cases, and zero non-registry regex literals

Scan and command flow:

• extend candidate workspace scans to JavaScript-compatible custom
  $TAGS regexes with balanced structure and zero-width tails
• keep PCRE2-only backreference regexes on raw ripgrep normalization
• wrap registered commands in a shared async boundary and return
  persistence, reveal, export, and migration work
• roll back workspace-state writes before refresh work when
  multi-entry writes fail

Ripgrep and evidence:

• return RipgrepError for option, executable, spawn, JSON, cleanup,
  and cancellation failures
• prevent stale search cleanup from clearing newer active searches
• add registry, scan parity, ripgrep, workflow, release, and perf
  coverage plus accuracy and candidate-scan artifacts
• ignore generated accuracy JSON and local settings exports

Fixes #51