…#24506)

## What

The mutation/aliasing inference interns every effect it applies. Both
the interning map and the fixpoint-stable ValueId cache were keyed by a
formatted `String`, so every lookup on the `apply_effect` hot path built
a fresh string. On top of that, `AliasingEffect::Apply` carried an owned
`FunctionSignature`, so cloning an Apply effect meant copying the whole
struct including its parameter effect list, and `apply_signature`
deep-cloned the cached effect list of every instruction it applied.

## Change

- Replace the `String` key with an `EffectKey` enum carrying exactly the
fields the string encoded. `Apply::signature`/`span` and
`Mutate::reason` still don't participate in identity, and the one
collision the string format had (an empty `Apply` argument list vs a
single hole) is reproduced on purpose. Building and hashing a key no
longer allocates or formats, except in the rare error-carrying arms,
which clone their message strings.
- Put the `FunctionSignature` in `Apply` behind `Rc`. It's still
deep-cloned out of the shape registry once per call instruction, but
every later clone of the effect is just a refcount bump. The interning
key already ignores the signature, and all readers are immutable.
- Put the cached `InstructionSignature` entries behind `Rc` too, so
`apply_signature` can hold the signature while passing the context on
mutably, instead of cloning the whole effect list each time.

Follow-up to #24117; this PR is based on current main, which includes
it.

Numbers from linting cal.com with only `react/react-compiler` enabled,
single thread, fat-LTO release build:

| Metric (cal.com, 1 thread) |     main |  this PR | delta |
| -------------------------- | -------: | -------: | ----: |
| instructions               | 19.113 G | 18.086 G | −5.4% |
| wall time                  |  2.907 s |  2.804 s | −3.5% |

## Verification

- Output of the main binary vs this PR's binary is byte-identical
(stdout, stderr, exit code) on all 4 corpus/config combinations: cal.com
and excalidraw, each with `reportAllBailouts` on and off.
- The `react_compiler` example (parse, compile, codegen) produces
identical output on all 1438 corpus files.
- `cargo test -p oxc_react_compiler` passes, including the 1,736-fixture
snapshot corpus: compiled output is unchanged on every fixture.
- `cargo clippy` and `cargo fmt` clean.

One note for future re-syncs with the vendored React Compiler sources:
`EffectKey` mirrors `AliasingEffect` variant for variant. Adding an
identity-relevant field to a variant without updating `effect_key`
silently changes interning behavior. Keep the two in sync.

This PR was assisted by Claude Code.

…ate merge (#24805)

## Context

Value sets in the aliasing inference state are `Rc<FxHashSet<ValueId>>`,
shared between states rather than copied (#24117). When two states meet
at a join, the merge still walks every variable and probes one set
against the other looking for a value the current state doesn't have.
Most of the time both sides point at the same `Rc`, so that probe is
comparing a set to itself.

## This PR

Check `Rc::ptr_eq` before probing. If the pointers match there is
nothing to add, so we skip the scan. Both merge paths did this check in
slightly different ways, so I pulled it into one helper. The sets are
never mutated in place, which is what makes the pointer check safe.

### Performance

| workload | main | this PR | delta |
| ----------------------------------------------------- | -------: |
-------: | ------: |
| 3.6k-line control-flow heavy component, instructions | 23.762 B |
22.034 B | -7.3% |
| same file, wall time |   4.55 s |   4.13 s |     -9% |
| 16.5k-file real corpus, instructions (1 thread) | 50.135 B | 49.697 B
| -0.87% |

This PR was assisted by Claude Code.