feat(core)!: Make last_active_indices a Vec<(K, u32)>

[evanlinjin]

Description

Fixes #2314.

FullScanResponse::last_active_indices is now a Vec<(K, u32)> instead of a
BTreeMap<K, u32>, and KeychainTxOutIndex::reveal_to_target_multi takes
impl IntoIterator<Item = (K, u32)> instead of &BTreeMap<K, u32>.

A chain source learns of activity one spk at a time, in arbitrary order, so a
map forces every producer to keep a running per-keychain max before it can
report (a plain insert there silently under-reports the last active index).
The consumer never needed a map: _reveal_to_target only reveals upwards, so
unordered and repeated entries are already handled correctly. Merging two
responses is now a concatenation instead of a per-key max.

Notes to the reviewers

The esplora and electrum producers now push each observation as they make it.
Both still emit at most one entry per keychain, so the tests assert on the exact
Vec.

Changelog notice

• Breaking: FullScanResponse::last_active_indices is now Vec<(K, u32)>.
• Breaking: KeychainTxOutIndex::reveal_to_target_multi now takes
  impl IntoIterator<Item = (K, u32)>.

Checklists

All Submissions:

• I followed the contribution guidelines

Bugfixes:

• This pull request breaks the existing API
• I'm linking the issue being fixed by this PR

🤖 Generated with Claude Code

https://claude.ai/code/session_01VATvTCqPsmqxZuvzm7WZ9s

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.85%. Comparing base (c6a6073) to head (3a6795e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2315      +/-   ##
==========================================
+ Coverage   78.84%   78.85%   +0.01%     
==========================================
  Files          31       31              
  Lines        6060     6063       +3     
  Branches      288      288              
==========================================
+ Hits         4778     4781       +3     
  Misses       1203     1203              
  Partials       79       79              

Flag	Coverage Δ
rust	78.85% <100.00%> (+0.01%)	⬆️

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