docs(core): document hashbrown feature's fixed-key hashing

[Romansage6]

Description

The hashbrown feature pulls in hashbrown with default-features = false
and neither its runtime-rng nor compile-time-rng feature enabled, so its
ahash hasher falls back to fixed, publicly known keys. Every hash map built
on it (e.g. TxGraph keyed by Txid, SpkTxOutIndex keyed by ScriptBuf)
therefore hashes identically across processes, machines, and builds. With
untrusted high-volume input (e.g. a chain source returning crafted entries)
this permits worst-case quadratic insertion, which can stall a sync. This
tradeoff wasn't documented anywhere.

Notes to the reviewers

Documented as a crate-level "Feature flags" rustdoc section in bdk_core,
where the feature is actually defined (crates/core/src/lib.rs), rather than
in the one-line README.md or in the collections module (which is
#[doc(hidden)] and wouldn't render). Also notes the mitigation: depend on
hashbrown directly with runtime-rng/compile-time-rng enabled, which
Cargo's feature unification propagates into this crate's internal use.

Changelog notice

• Document that the hashbrown feature uses fixed hash keys, making hashing
  deterministic and exposing untrusted high-volume input to worst-case
  quadratic hash-map insertion.

Checklists

All Submissions:

• I followed the contribution guidelines

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

Fixes #2342

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.84%. Comparing base (2c89f60) to head (93ac7c3).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

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  Coverage   78.84%   78.84%           
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  Files          31       31           
  Lines        6060     6060           
  Branches      288      288           
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  Hits         4778     4778           
  Misses       1203     1203           
  Partials       79       79           

Flag	Coverage Δ
rust	78.84% <ø> (ø)

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