test(runtime): an absent-key read may publish its holder entry (#11681)

[proggeramlug]

Fixes #12207.

an_unresolved_cache_slot_is_never_dereferenced has failed since #11681 (7fa094c). I bisected it by running this one test: it passes at #11681's parent and fails at #11681 and at every later commit tried. This PR fixes the test, not the runtime.

#11681 made read sites answer absent keys from holder-shape facts on purpose. On a NotOwn miss, prime_read_holder publishes an absent holder entry, and that resolves the slot. The test still asserted the pre-#11681 rule that an absent read leaves the slot null.

The entry is validated against the receiver and hop shapes on every use, so all of these invalidate it:

• adding the key to the receiver or to any prototype;
• delete;
• setPrototypeOf on the receiver or on a hop;
• defining an accessor on a prototype.

A Proxy in the chain is never admitted. Existing tests that cover this:

• memo_tests::alternating_terminals_and_deep_hops_hit_and_invalidate_independently;
• class_read::tests::multi_absent_*.

Changes:

• Drop the slot.is_null() assert.
• Keep the guarantees the test is about: the read returns undefined and leaves the own-key ways unprimed (packed == 0).
• Add a second read through the same slot, which must also return undefined.
• Update the doc comment.

Results: cargo test --release -p perry-runtime --lib -- --test-threads=1 ic_slow passes 24 tests with 0 failures.

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• crates/perry-runtime/src/object/field_get_set/ic_miss/ic_slow.rs

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