Clarify resource limits for prebuilt trees

[jskoiz]

Summary

• Clarify that the documented resource limits apply while parsing parser-backed input.
• Document that from_node, from_value, and direct tree deserialization do not re-run LoadOptions parser limits for caller-built Node/Value trees.
• Add a regression test covering both caller-built tree entry points against a parser-enforced collection limit.

Fixes #84

Verification

Local proof:

• CARGO_INCREMENTAL=0 CARGO_BUILD_JOBS=1 cargo test --locked --quiet — passed all unit, integration, fixture, and doctest suites.
• cargo fmt --all --check and git diff --check — passed.
• CARGO_INCREMENTAL=0 CARGO_BUILD_JOBS=1 cargo clippy --locked --all-targets -- -D warnings — passed.
• CARGO_INCREMENTAL=0 CARGO_BUILD_JOBS=1 scripts/check-feature-clippy.sh — passed.
• CARGO_INCREMENTAL=0 CARGO_BUILD_JOBS=1 RUSTDOCFLAGS='-D missing_docs' cargo doc --locked --no-deps — passed.
• scripts/check-public-api.sh — passed with no public API snapshot change.
• mdbook build — passed.

Hosted/merge state:

• Branch c1/issue-84-prebuilt-tree-limits is pushed to origin.
• This PR is intentionally draft.
• Hosted CI has not yet been observed from this PR, and the PR has not been merged.

Notes/Risks

• This is a documentation clarification plus regression coverage; it does not add an options-aware validator for caller-built trees.
• The separate << merge-expansion depth guard remains in place, but it is not a general resource-limit validation pass.