fix(templates): accept dotted VictoriaLogs fields in templates (#25)

[fxthiry]

Summary

Fixes #25. VictoriaLogs emits fields like nginx.http.request_id as literal flat keys, but minijinja interprets a.b.c as nested attribute access — so both valerter --validate and runtime rendering fail on user-written templates referencing these fields.

Two complementary fixes:

• At render time — TemplateEngine now deflates the event additively before handing it to minijinja. Flat dotted keys stay accessible, and a nested structure is added on top so {{ nginx.http.request_id }} resolves. Deep-merges with existing nested values; on collision with an existing top-level scalar, the scalar wins and a warn! is emitted.

• At validation — the validate_template_render sentinel is swapped from json!({}) to a TruthyChainable Object: returns itself on any attribute access, always truthy, iterates as an empty sequence, stringifies as empty. Keeps {% if x.y %} bodies walked so unknown-filter detection does not regress, while allowing chained undefined access and {% for %} / | length against undefined to validate cleanly.

Test plan

• cargo test --lib — 429 passed (baseline 427 + 2 new regression tests)
• cargo clippy --all-targets --all-features -- -D warnings — clean
• Manual repro on the exact config from [Bug]: Error warning when fields contain dot characters, sample nginx.http.request_id #25 — valerter --validate returns Configuration is valid (exit 0)
• Smoke-test on reporter's real VictoriaLogs instance (via release/1.1.1 rc build)

Review hints

Spec and suggested review order: _bmad-output/implementation-artifacts/spec-gh-25-dotted-field-templates.md (local, not in VCS).

Key entry points:

• src/config/validation.rs:20 — TruthyChainable sentinel (the design decision)
• src/parser.rs:271 — unflatten_dotted_keys (additive deflation)
• src/template.rs:165 / src/template.rs:180 — application points before render_str

Targeted at release/1.1.1 rather than main so the patch release can be bundled with #26 and validated by the reporter before tagging.