Fix ChangeSet write-once invariants

[Copilot]

Description

ChangeSet's three write-once fields (descriptor, change_descriptor, network) had inconsistent invariant enforcement: the Merge impl's debug_assert! was a no-op in release builds, and the SQLite upsert unconditionally overwrote stored values.

Changes

Merge impl (src/wallet/changeset.rs)

• Changed from "always overwrite if incoming is Some" to "only accept if existing is None" (first-write-wins)
• debug_assert! moved to the else branch — still catches violations in dev builds

// Before: silently overwrites in release
if other.descriptor.is_some() {
    debug_assert!(...);
    self.descriptor = other.descriptor;
}

// After: first-write-wins, assert catches conflicts in debug
if self.descriptor.is_none() && other.descriptor.is_some() {
    self.descriptor = other.descriptor;
} else {
    debug_assert!(other.descriptor.is_none() || self.descriptor == other.descriptor, ...);
}

SQLite persistence (persist_to_sqlite)

• Replaced DO UPDATE SET field = :field with DO UPDATE SET field = COALESCE(bdk_wallet.field, :field) for all three write-once columns. COALESCE returns a copy of its first non-NULL argument, or NULL if all arguments are NULL.
• Mirrors first-write-wins at the SQL layer; safe across multiple independent upsert statements (one per field) since each only touches its own column

Tests

• merge_first_write_wins: covers None+Some, Some+None, Some+Same scenarios, gated by #[cfg(not(debug_assertions))]

Fixes #470

Changelog notice

Fixed

• ChangeSet::merge enforces descriptor and network invariants by ensuring only the first write is retained.
• Fixed an issue in ChangeSet::persist_to_sqlite that would have allowed changing the descriptor or network in the wallet database

Before submitting

• I followed the contribution guidelines
• This PR breaks the existing API

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.77%. Comparing base (20349c7) to head (9a75129).

Files with missing lines	Patch %	Lines
src/wallet/changeset.rs	33.33%	6 Missing and 6 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #517   +/-   ##
=======================================
  Coverage   81.77%   81.77%           
=======================================
  Files          25       25           
  Lines        6484     6487    +3     
  Branches      296      296           
=======================================
+ Hits         5302     5305    +3     
+ Misses       1083     1080    -3     
- Partials       99      102    +3     

Flag	Coverage Δ
rust	81.77% <33.33%> (+<0.01%)	⬆️

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