B::svref_2object($cv)->GV->NAME previously returned "__ANON__" for CVs
renamed via Sub::Name::subname or Sub::Util::set_subname, because B.pm
required `defined &{$fqn}` to trust the name. Real-Perl XS Sub::Name
updates the CV's CvGV/CvNAME_HEK directly and B reads those fields
without consulting any stash entry, so the name is honored even when
the sub was never installed as a glob.

The `defined &{$fqn}` check was originally added to handle stash
deletion/clearing (op/stash.t, uni/stash.t), so it cannot be removed
outright. Instead, track an explicit `explicitlyRenamed` flag on
RuntimeCode that is set by Sub::Name::subname and Sub::Util::set_subname,
and have B.pm trust the name whenever that flag is set.

- RuntimeCode: add `explicitlyRenamed` field
- SubName.java / SubUtil.java: set the flag when renaming
- SubName: expose private helper `Sub::Name::_is_renamed($cv)`
- B.pm: consult the flag before falling back to the stash check

Impact on Sub-Name-0.28 t/exotic_names.t: 1038 more tests now pass
(0 -> 1038 pass out of 1560). Remaining 522 failures are unrelated to
Sub::Name and stem from stash-aliasing semantics
(`*palatable:: = *{"aliased::native::..."}`).

No regressions in op/stash.t / uni/stash.t (75/105 before and after).

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Makes `*Dst:: = *Src::;` actually alias the namespaces at both the
storage and lookup levels, matching Perl 5 semantics — while preserving
the existing compile-time-qualified reference behaviour (`&Pkg::foo`
keeps pointing at its original CvGV, as real Perl does).

Before:
  *Dst:: = *Src::;
  \%Dst:: == \%Src::        # false  (!!!)
  sub Dst::foo {};
  defined &Src::foo         # false  (!!!)

After:
  *Dst:: = *Src::;
  \%Dst:: == \%Src::        # true
  eval 'sub Dst::foo {}';
  defined &Src::foo         # true
  *{"Dst::sym"} = sub {};
  defined &Src::sym         # true
  ${"Dst::var"} = 42;
  ${"Src::var"}             # 42
  # transitive chains also collapse:
  *Mid:: = *Src::;
  *Fin:: = *Mid::;
  \%Fin:: == \%Src::        # true

Implementation (see dev/prompts/stash-aliasing-plan.md for the design):

1. GlobalVariable infrastructure
   - resolveAliasedFqn(fqn): resolves the package prefix of an FQN
     through stashAliases. Fast path when the alias map is empty (no
     hashing, no substring) — this is the overwhelmingly common case.
   - resolvedStashAliasCache: memoised transitive resolution with a
     16-hop cycle cap. Invalidated on setStashAlias/clearStashAlias/
     resetAllGlobals. Non-aliased packages map to their own String
     instance so callers can shortcut the substring+concat via
     reference equality (`resolved == pkg`).

2. Hash storage unification in RuntimeGlob.set(RuntimeGlob)
   When both glob names end with "::", also put the source's
   RuntimeStash into globalHashes under the destination key. This
   makes `\%Dst:: == \%Src::` true and `*Dst::{HASH} == *Src::{HASH}`
   true.

3. Resolve-with-fallback on read-side accessors
   getGlobalVariable / getGlobalArray / getGlobalHash / getGlobalCodeRef
   and their `exists*` siblings first try the alias-resolved FQN; if
   no value lives there, they fall back to the raw key. This preserves
   compile-time-qualified references like `\&MCTest::Base::foo` that
   point at CVs still living at their original FQN, matching real Perl
   (which keeps each CV's CvGV pinned to the package it was compiled
   in — the alias only redirects new writes and runtime symbolic refs).

4. Resolve-on-install for sub declarations
   defineGlobalCodeRef always resolves through the alias so that
   `*Dst:: = *Src::; sub Dst::foo {}` installs the sub under Src::foo.
   SubroutineParser applies resolveAliasedFqn to the computed fullName
   before splitting into code.packageName / code.subName, so caller()
   and set_subname report the correct target package.

5. SubroutineParser package/name split bugfix
   placeholder.subName was being set from the raw subName parameter,
   which may contain "::" (parsing `sub Dst::foo {}` arrives with
   subName="Dst::foo"). Combined with alias resolution rewriting the
   full name to "Src::foo", this produced code.subName="Dst::foo"
   and code.packageName="Src" and caller() reported "Src::Dst::foo".
   Derive both halves from the resolved fullName instead.

Note: resolveAliasedFqn is *not* invoked inside NameNormalizer. That
route was too aggressive — it rewrote compile-time-qualified reads
like `\&MCTest::Base::foo`, causing a regression in
perl5_t/t/mro/method_caching.t where the sub still exists at its
original FQN after aliasing. The resolve-on-install + resolve-then-
fallback-on-read split avoids this.

Known limitation
----------------
GV-level aliasing — where a stash hash key stores a GV whose own name
differs from the hash key, e.g.:
  ${"palatable::"}{"sub"} = ${"palatable::"}{$encoded_sub};
is not supported. PerlOnJava's flat-map storage can't represent
"hash key X points to GV named Y" because globs are currently just
FQN strings without a separate name field. Tracked as follow-up in
the plan doc; it's what's blocking the remaining 392 failures in
Sub-Name-0.28 t/exotic_names.t.

Testing
-------
- New src/test/resources/unit/stash_aliasing.t with 5 subtests
  (hash identity, sub installation, caller reporting, symbolic refs,
  chained aliases) — all pass.
- make: all unit tests pass.
- perl5_t/t/op/stash.t + perl5_t/t/uni/stash.t: 75/105, unchanged.
- perl5_t/t/mro/method_caching.t: 31/36, unchanged from master.
- perl5_t/t/mro/method_caching_utf8.t: 28/28, unchanged from master.
- Sub-Name-0.28 t/exotic_names.t: +130 tests (1038 -> 1168 / 1560),
  cumulative with the B/Sub::Name fix in the preceding commit.

Design doc: dev/prompts/stash-aliasing-plan.md (new)

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>