test: add comprehensive unit tests for Template::Document

[toddr-bot]

What

106 unit tests for Template::Document, covering the full public API and internals.

Why

Template::Document is the core compiled-template representation — every template render creates and processes one. No dedicated unit tests existed; coverage came only indirectly through integration tests.

How

Tests cover:

• Constructor: coderef blocks, string blocks (eval'd), invalid blocks, DEFBLOCKS (coderefs, strings, mixed), METADATA, VARIABLES, defaults
• Accessors: block(), blocks(), variables(), meta() (filtering rules for _-prefixed keys, name, modtime)
• AUTOLOAD: metadata access, nonexistent keys
• process(): visit/leave lifecycle, context passing, _HOT recursion detection (with and without RECURSION flag), error propagation with cleanup
• as_perl(): output structure, metadata escaping, roundtrip eval, version stamp
• write_perl_file(): file creation, do-loadable output, invalid filenames, UTF-8 handling
• catch_warnings(): accumulation behavior
• Inheritance: Base class integration

Testing

perl -Ilib t/unit-document.t  # 106 tests pass
make test                      # full suite green

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Quality Report

Changes: 1 file changed, 668 insertions(+)

Code scan: 1 issue(s) found

• t/unit-document.t:0 — 668 lines added

Tests: passed (OK)

Branch hygiene: clean

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