Closes #532.

Adds ReScript as a first-class indexed language, following the existing per-language line-parser pattern (mirrors how .r #215 and .tf #108 were added).

What

• Detection: .res and .resi → new rescript language.
• parseRescriptLine extracts:

  ReScript	Symbol kind
  let f = (…) => … / let rec / and f = … =>	function
  let x = value (no arrow)	constant
  type t = …	type_alias
  module M = { … }	struct_def
  module type S = { … }	interface_def
  external name: T = "…"	function
  open M / include M	import

• Leading @decorator / @decorator(args) are stripped first, so a decorated @val external … or @react.component\nlet make still resolves.
• // and /* */ comments are skipped (ReScript joins the C-style block-comment group + isCommentOrBlank). langHasCallSites/isCheckableCode fall through their else arms correctly (call-sites: yes, delimiter-balance check: no).

Tests (failing-first, per CLAUDE.md)

Commit 1 adds issue-532 tests that fail on the base branch (.res → .unknown, empty outline); commit 2 implements support and turns them green:

• test_parser.zig — full outline of a .res file (module/type/let/external/open).
• test_explore.zig — detectLanguage for .res/.resi.

zig build test passes clean (0 failures).

End-to-end (codedb outline on a real .res)

✓ src/User.res  rescript  25 lines  ⚡ 4.0µs
  L1   import      Belt
  L3   struct_def  User
  L4   type_alias  t
  L9   function    make
  L11  function    greet
  L14  type_alias  status
  L18  constant    defaultName
  L20  function    fib
  L22  function    setTimeout   (decorated @val external)
  L24  function    helper       (and-binding)

Known limitations (v1, additive)

Line-based like the sibling parsers: nested module members are flattened into the top-level outline, and exception/variant constructors are not yet emitted as symbols. These can be follow-ups; nothing here is a filter, so recall is never reduced.

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