test: add comprehensive unit tests for Template::Filters

[toddr-bot]

What

290 assertions across 51 subtests exercising every public method, static filter,
dynamic filter factory, and helper function in Template::Filters.

Why

Existing t/filter.t tests filters through template processing (DATA-driven).
No direct unit tests existed for the Filters module API — constructor options,
fetch/store pipeline, error handling, TOLERANT mode, or individual filter
functions called directly.

How

• Mock context/stash for dynamic filters that need one
• Test constructor with defaults, custom FILTERS, TOLERANT, and DEBUG
• Test fetch() for static, dynamic, coderef pass-through, unknown names,
  TOLERANT downgrade, and various error conditions (bad factory, dying factory)
• Test store() and round-trip fetch of stored filters
• Direct tests of all 16 static filters (html, xml, uri, url, upper, lower,
  ucfirst, lcfirst, trim, collapse, null, stderr, html_para, html_para_break,
  html_line_break)
• Direct tests of all 8 dynamic filter factories (indent, format, truncate,
  repeat, replace, remove, eval, perl) plus redirect and stdout
• HTML entity-aware truncation helpers (_visual_length, _truncate_visual)
• RFC2732/RFC3986 toggle behavior, UTF-8 URI encoding
• Registry completeness check and alias verification
• Package variable defaults

Testing

perl -Ilib t/unit-filters.t   # 290 assertions, 51 subtests, all pass
make test                      # full suite passes

---

Quality Report

Changes: 1 file changed, 679 insertions(+)

Code scan: 1 issue(s) found

• t/unit-filters.t:0 — 679 lines added

Tests: passed (OK)

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline