- Adds `docs/reference/advanced/managed-variables/templates-and-composition.md`
  covering Handlebars `{{placeholder}}` rendering via `logfire.template_var()`
  and `@{variable_name}@` composition references, including structured
  values, cycle detection, and how templates and composition combine.
- Expands the managed-variables index with a templates intro and links
  to the new page.
- Mentions `template_inputs_schema` and the `[variables]` extra in the
  configuration reference and nav.
- Adds `examples/python/variable_composition_demo.py` exercising
  composition, structured variables, template inputs, and
  composition-time conditionals end-to-end.
- Skips Python doc examples whose source mentions `logfire.template_var`
  when `pydantic-handlebars` is unavailable (matches the runtime
  requirement on Python 3.9).

Replaces the regex-and-translate workaround in reference_syntax.py and
composition.py with calls into pydantic-handlebars >= 0.2.0's native
configurable-delimiter API.

## What changed

- pyproject.toml: bump `pydantic-handlebars` to `>=0.2.0` (in both the
  `[variables]` extra and the dev group) and exempt it from the
  `exclude-newer` filter alongside the other Pydantic packages.
- `_handlebars.get_environment()` returns a cached
  `HandlebarsEnvironment(open_delim='@{', close_delim='}@')` for the
  composition pass. `extract_composition_dependencies()` wraps
  `pydantic_handlebars.extract_dependencies` with the same delimiters.
- `reference_syntax.render_once()` shrinks to a one-line call into that
  environment. The sentinel-protect-then-regex-translate code path is
  gone; `{{...}}` runtime placeholders survive because they're plain
  content under the configured delimiters.
- `composition.find_references()` / `_collect_ref_names()` now route
  through `extract_composition_dependencies` for AST-correct detection,
  then a textual position scan supplies first-occurrence ordering.

## What you can now write

`@{...}@` accepts the full Handlebars syntax — block helpers with
dotted-path headers, `{{#each}}` parent-context references (`../`),
helper subexpressions, the lot. Previously the regex translator only
covered top-level identifiers in block headers, so e.g.
`@{#if user.active}@` silently dropped its condition. New regression
tests cover these in `TestExpandReferencesNativeHandlebarsSyntax` and
`TestFindReferencesNativeHandlebarsSyntax`.

The unresolved-dotted-reference protection in `_render_value` is kept
for behaviour compat (literal `@{name.field}@` is retained in the
output when `name` can't be resolved). That's the
"misrender retention" parity question — addressed separately.

- Use exact, trimmed equality against 'null' in expand_references
  rather than a case-insensitive startswith match. JSON null is
  always lowercase, and 'nullify' shouldn't have matched.
- Add a test that exercises the 'dotted ref whose base is resolved'
  branch in _protect_unresolved_dotted_refs, which the regex
  narrowing in this PR left uncovered.

…le-composition-native-handlebars

…tic/logfire into feature/variable-composition-native-handlebars

Composition and render failures already warn before falling back to the
code default, but a value that was fetched/composed/rendered successfully
and then failed validation (including a context override that renders to
an invalid value) fell back silently. Emit a RuntimeWarning in both paths
so the substitution is observable, closing the asymmetry Alex flagged on
#1954 (tests/test_variable_templates.py:549 / #1951 discussion r3289312130).