Wasm size optimization round 2 (post-feature target 30% reduction)

[truthixify]

Tier: M (2-4 days) Type: perf

Context

Several feature PRs landed since the last size review (#41 v2 announcer, #30 on-behalf flow, #32 social recovery, #42 splitter). Need a second pass on size + a CI gate to keep us under Soroban's upload budget.

Scope

• Re-measure all four contracts (debug, release, post stellar contract optimize)
• Identify the two largest growth sources since the last baseline in stellar/SIZE.md
• Land at least one optimization (binary size only, no semantic changes)
• Add CI gate that fails if any contract exceeds 110 KB

Acceptance criteria

• Updated stellar/SIZE.md with before/after numbers
• CI size-budget job in place
• At least one landed optimization with measured win
• Documented "what to do if you exceed budget" recipe

Files to start with

• stellar/SIZE.md (existing baseline)
• stellar/*/Cargo.toml (per-crate profile)
• stellar/Cargo.toml (workspace profile)