Enforce collection limits after merge expansion

[jskoiz]

Summary

• Propagate the active LoadOptions into semantic merge expansion.
• Enforce collection limits before and after merge materialization so effective mappings cannot bypass max_collection_items.
• Preserve strict/default merge precedence, YAML 1.1 literal recovery, and raw-event merge syntax.
• Add focused regression coverage for semantic loaders, raw events, alias-limit precedence, and YAML 1.1 behavior.

Verification

Local proof:

• cargo fmt --all --check — passed.
• git diff --check — passed.
• cargo test --locked --test dos_hardening --test merge_defaults — attempted before implementation; compilation stopped with No space left on device. Per the workspace disk constraint, no further Cargo build was run after the fix.

Hosted state:

• Draft PR opened against main.
• Hosted CI has not been observed yet and remains pending.
• No merge was performed.

Notes/Risks

• Scope is limited to issue [P1] Enforce collection limits after merge expansion #79.
• The worktree filesystem constraint prevented post-change compilation/test confirmation; hosted CI or a later local Cargo run should provide that gate.
• Raw event/lossless surfaces remain unexpanded, while semantic paths enforce the effective collection limit.