DD-061 PR1: Align template contract and docs

[larimonious]

Summary

• Route template expression/filter/condition failures through the existing TypeMode template boundary
• Keep undefined template variables falsy/empty for optional slots, but fail closed in strict mode for real errors
• Update generated and human docs to describe NTNT template syntax, explicit #if/#for blocks, partial resolution, and literal delimiter escapes

Verification

• cargo fmt
• cargo build --profile dev-release
• cargo test --test language_features_tests honor_type_mode -- --nocapture
• ./target/dev-release/ntnt docs --generate
• cargo test
• git diff --check

Notes

PR1 for DD-061: no caching or broad template redesign; this is the contract/docs cleanup slice.

[greptile-apps]

Greptile Summary

This PR aligns NTNT template handling with the documented template contract. It changes:

• Routes template expression, filter, and condition failures through the TypeMode template boundary.
• Keeps undefined template variables falsy or empty for optional slots.
• Updates template docs for explicit #if and #for blocks, partial resolution, and literal delimiter escapes.
• Bumps the package and generated docs version to 0.4.12.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

_{Reviews (2): Last reviewed commit: "Bump version to 0.4.12" | Re-trigger Greptile}