feat(resolver): support inline #ref pin and default-branch alias

Adds the missing pieces for installer fixtures that pin a dev package
via `dev-foo#hex` or rely on Composer's `default-branch: true` synthetic
`9999999-dev` alias.

Mirrors Composer at four layers:

1. `mozart_semver::parse_single` strips `dev-...#hex` / `....x-dev#hex`
   suffixes from constraints (Composer's `parseConstraint` regex).
2. `PackagistVersion` carries `default_branch`. When set on a `dev-`
   package with no numeric prefix, `packagist_to_pool_inputs` emits
   the synthetic `9999999-dev` alias — but skips it when an explicit
   `extra.branch-alias` already covers the version (matches
   `ArrayLoader::getBranchAlias`).
3. `RuleSetGenerator::generate` picks up `addRulesForRootAliases`:
   any pool alias whose target was added gets its own alias↔target
   rules so the SAT solver pulls them in together.
4. `lockfile::generate_lock_file` extracts root `#hex` overrides from
   `require`/`require-dev` and rewrites source/dist references (and
   github/gitlab/bitbucket archive URLs) on the matched package, the
   `setSourceDistReferences` ladder Composer runs in `PoolBuilder`.

Resolver also infers `Stability::Dev` from a `dev-foo` style
single-atom constraint when no explicit `@flag` is given, mirroring
the second loop of `RootPackageLoader::extractStabilityFlags` so the
package isn't filtered out under default `stable` minimum-stability.

Newly green: install_branch_alias_composer_repo, install_reference,
conflict_with_alias_prevents_update_if_not_required,
unbounded_conflict_matches_default_branch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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