feat(stash): alias storage + name resolution for *Dst:: = *Src::

[fglock]

Summary

Implements Commits 1–3 of dev/prompts/stash-aliasing-plan.md to make *Dst:: = *Src::; actually alias the namespaces, matching Perl 5 semantics.

⚠️ Stacked on top of #541 (Sub::Name B::GV fix). Merge #541 first; GitHub will auto-retarget this PR to master when that happens.

What now works

package Src; sub hello {}
package main; no strict 'refs';
*Dst:: = *Src::;

\%Dst:: == \%Src::          # true
keys %Dst::                  # ['hello']
eval 'sub Dst::foo {}';
defined &Src::foo            # true
*{"Dst::sym"} = sub {};
defined &Src::sym            # true
${"Dst::var"} = 42;
${"Src::var"}                # 42
# chained:
*Mid:: = *Src::;
*Fin:: = *Mid::;
\%Fin:: == \%Src::           # true

Changes

1. GlobalVariable infrastructure. New resolveAliasedFqn(fqn) plus a memoised transitive-resolution cache resolvedStashAliasCache with cycle cap. Fast path when stashAliases.isEmpty(); non-alias hits return the input string identity so callers can skip the substring+concat via ==. Invalidated on every setStashAlias/clearStashAlias/resetAllGlobals.

2. Hash unification in RuntimeGlob.set(RuntimeGlob). When both glob names end with ::, also globalHashes.put(this.globName, getGlobalHash(value.globName)) so the two stash-view hashes share storage.

3. NameNormalizer.normalizeVariableName applies resolveAliasedFqn after the existing cache, covering symbolic refs, eval "sub Dst::foo {}", and ${"Dst::var"}.

4. SubroutineParser package/name split bugfix. placeholder.subName was being set from the raw parser token (may contain ::, e.g. "Dst::foo"), while packageName was derived from the alias-resolved fullName. caller() then concatenated to Src::Dst::foo. Now both halves are derived from the resolved fullName.

Caching strategy

resolveAliasedFqn is on the hot path for every global symbol access. Two layers protect it:

• Empty-map fast path. if (stashAliases.isEmpty()) return fqn; — O(1) for programs that never use stash aliasing (i.e. almost all programs).
• Transitive cache. Once an alias exists, each "Pkg::" maps in resolvedStashAliasCache to its terminal target. Chains collapsed once, reused forever (until a mutation clears the cache). Non-aliased packages map to their own string instance so the caller uses reference equality to shortcut the concat.

Test Impact

• Sub-Name-0.28 t/exotic_names.t (the test that surfaced this): 1038 → 1168 passing (out of 1560) cumulative since fix: Sub::Name B::GV introspection + stash aliasing for *Dst:: = *Src:: #541.
• Remaining 392 failures are all the "natively compiled sub" subtest that uses GV-level aliasing (${"palatable::"}{"sub"} = ${"palatable::"}{$encoded_sub}) — requires GVs with their own name field independent of the stash slot; deeper refactor, tracked in the plan doc.
• perl5_t/t/op/stash.t + perl5_t/t/uni/stash.t: 75/105 → 75/105 — no regression.
• make unit tests: all pass, including a new src/test/resources/unit/stash_aliasing.t covering hash identity, sub installation through alias, caller() reporting, symbolic refs, and chained aliases.

Test plan

• make (unit tests)
• New src/test/resources/unit/stash_aliasing.t — 5 subtests, all pass
• perl dev/tools/perl_test_runner.pl perl5_t/t/op/stash.t perl5_t/t/uni/stash.t — 75/105, unchanged
• jcpan -t Sub::Name — exotic_names.t 1038→1168 passing

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[fglock]

Unified into #541 — stacked PR no longer needed; #541 now contains both commits (Sub::Name B::GV fix + stash aliasing).