regex: add --regextype=SHGLOB (GLOB + {a,b} brace alternation) (#129)

[helly25]

What

A new match grammar --regextype=SHGLOB ("shell glob"): everything GLOB does, plus shell brace alternation, so *.{cc,h} matches either extension. As a matcher (not a shell) a brace group compiles to an RE2 alternation - *.{cc,h} -> [^/]*\.(?:cc|h) - it matches any one alternative rather than expanding to several words.

Why a separate grammar (not a GLOB feature)

GLOB and the gitignore engine must keep matching literal {/} (a .gitignore line foo{1} matches literally), so brace expansion has to be opt-in. Named SHGLOB, deliberately not EXTGLOB - in bash that names the pattern-lists ?(..)/@(..)/!(..), which we do not ship (the !(..) negation has no clean RE2 form).

Semantics (match bash)

pattern	RE2	note
*.{cc,h}	[^/]*\.(?:cc|h)	the motivating case
{src,test}/**/x	(?:src|test)/(?:.*/)?x	alts contain / and **
{a,{b,c}d}	(?:a|(?:b|c)d)	nesting is recursive
{a,,b}	(?:a||b)	empty alternatives allowed
{a}	\{a\}	no top-level comma -> literal
\{a,b\}	\{a,b\}	\{ \} \, escape to literals

Deferred: numeric/char sequences {1..9}/{a..z}, and the extglob pattern-lists.

Implementation (DRY)

xff::glob gains an internal flavor-aware TranslateInto shared by GlobToRegex (braces off) and the new ShglobToRegex (braces on, recursive {...} parse); the **/[...] logic (incl. the #128 class scanner) is not duplicated. Wired through Grammar::kShglob (regex Compile), parser GrammarFromGlobals, run.cc ValidateRegextype (SHGLOB is a core engine, always linked), and the globals/doc-renderer SOT.

Self-documentation (+ the PCRE2 note)

--regextype gains a full details body documenting every grammar. It also fixes a gap: the help always listed PCRE2 but never noted it may be absent. Now the details state PCRE2 is the one build-time extra, present only in a full build, and point to xff --help=extras for whether THIS binary has it (RE2/EXACT/FNMATCH/GLOB/SHGLOB are always built in). Flows into --help=--regextype / --help=full / --man / --markdown.

Also resolves #122

glob.h documents why xff keeps its own gitignore-** translator rather than migrating onto mbo::file::Glob2Re2 (different **; xff walks its own VFS engine, needs only the pure pattern->RE2 step).

Test

• glob_test: ShglobToRegex alternation/nesting/degenerate cases + a GLOB-keeps-braces-literal guard.
• regex_test: kShglob compile + match + retained GLOB path semantics.
• grep_test.sh: --regextype=SHGLOB end-to-end ({TODO,FIXME}) + GLOB literal-brace.
• Full //xff/... green.

Closes #129. Resolves #122.