Fix correctness, robustness, and tooling issues from code review

Fix correctness, robustness, and tooling issues from code review

Render: escape `<` to < in the embedded JSON so a `</script>` in any
string (e.g. a Haddock comment) can't close the data <script> early or
inject markup.

viewer.js:
  - Haddock markdown links: allowlist http(s)/mailto/relative targets so a
    `javascript:`/`data:` URL in a doc comment can't become a clickable XSS.
  - reduceTypeArg: iterate the family index (which folds in closed-family
    `tfEquations`) instead of only `pdFamInstances`, so closed-family
    applications actually reduce instead of showing as unresolved.
  - matchTypeArg: one-sided matching — a head variable is polymorphic and
    matches anything, but a variable left on the requirement side is
    abstract and no longer matches concrete instances (kills false
    "satisfied" edges).
  - Instance-view node ids use the stable global _idx so the focused and
    superclass-matched representations of one instance collapse to a single
    node and a filtered-out instance can't reappear under a second id.
  - escapeAttr now delegates to escape (one implementation).

Extract:
  - Data-family constructor arg types are substituted from the rep TyCon's
    tyvars to the fam-instance's, so positional TyVarRefs resolve instead of
    degrading to OtherArg.
  - Representational equality renders as the `Coercible a b` constraint
    (prefix) rather than infix.
  - Drop the always-Just Maybe from extractClass.

Merge:
  - Sort dumps by (package, module) before merging so dedup ("first wins")
    and output ordering are reproducible across filesystems.
  - normalisePackageId: treat a segment as a version only when it is
    digits-and-dots, not merely digit-led (avoids truncating names like
    base16-bytestring).

Plugin: document the purePlugin incremental-build behavior and the
clean-build caveat.

classgraph-plugin-flag.sh: collect every candidate .so (local build tree +
each cabal store that actually registers classgraph), offer a numbered
chooser when more than one is found, and stop an expected ghc-pkg failure
from tripping `set -e`.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>