sercon v0.5.1

Second Moderate cut. api.preg.* — PHP-style /pattern/flags
regex syntax on top of Go's stdlib regexp (RE2). The "preg"
naming is a deliberate homage; the semantics are RE2's. No new
dependencies.

Added

• api.preg.match(pattern, subject) — first hit as
  { match, groups, index }, or null when nothing matches.
  Optional groups that didn't match surface as empty strings (not
  undefined) so the result shape stays stable across calls.
• api.preg.matchAll(pattern, subject) — same shape, drained
  into an array.
• api.preg.replace(pattern, replacement, subject) — substitutes
  via Go's $1 / ${1} backref syntax. PHP's \1 form is not
  translated — \\ escapes are already legitimate in the
  replacement and aliasing them would surprise users coming from
  Go's regexp.ReplaceAllString.
• Supported flags: i / m / s, merged into Go's (?ims)
  inline-flag prefix. Unsupported PHP flags (u, U, x) and
  unknown flags throw with a clear, named-flag error rather than
  silently dropping. The u error explicitly notes that RE2 is
  UTF-8 by default so the flag is unnecessary.
• TestPreg* (15 sub-tests covering null-on-no-match, first-match
  groups + index, matchAll length, replace backrefs, all three
  supported flags, four unsupported / unknown flag forms, three
  malformed-pattern forms, optional-group empty-string).
• examples/scripts/preg.ts walks through every form above plus
  the unsupported-flag error path. Included in make demo and the
  CI offline subset (the binding is pure stdlib so it always
  works).
• --examples step 29 covers the binding; MANUAL section 5 gains
  the api.preg block plus a paragraph on the RE2-vs-PCRE
  semantics difference and the flag rules.
• cmd/sercon/api_docs.go grows three entries so the emitted
  api.d.ts carries hover docs for match / matchAll /
  replace.

Changed

• OUT-OF-SCOPE.md's entry for preg_match / preg_replace is
  rewritten as a forward-looking note about a possible
  regexp2-backed PCRE binding — RE2 covers most uses but
  scripts that genuinely need lookahead / lookbehind / pattern
  backrefs would justify a sibling api.preg2.* namespace.