feat: variadic --use-templates-variable for namespaced template sets

[crhntr]

resolves: #67

Why

*template.Template keeps every {{define}} name in a single global namespace. As an app grows or you try to share templates across projects, that becomes a bottleneck:

• Two halves of the same app (admin vs public) can't both define {{define "header"}} without one silently overwriting the other.
• A reusable component published as a Go module can't ship its own templates without imposing a globally unique naming convention on every consumer.
• All templates loaded into a single variable are forced to share one Funcs map, one set of Options, and one embed.FS.

The workaround today is to set every output flag and run muxt generate per package, but that leaves TemplateRoutePaths incomplete (each generated file only knows its own routes) and pushes duplicate-pattern detection to runtime in http.ServeMux.

Approach

--use-templates-variable becomes variadic. Each variable is the root of an independent namespace and contributes routes to a single combined TemplateRoutes function. Cross-variable duplicate route patterns are caught at generate time. TemplateRoutePaths covers everything because it sees all routes in one pass.

The user-facing rationale lives in docs/reference/templates-variable.md (Multiple Template Variables section).

Test plan

• go test ./..., go vet ./..., staticcheck ./..., gofumpt -d . — all clean
• txtar coverage for the happy path, cross-package embed.FS, cross-variable duplicate-pattern detection, per-variable unused-template detection, missing variable, and the deprecated/new flag mutex
• Unit coverage for fixTemplateVariables / findDuplicateVariables and CheckForDuplicatePatterns (method/host/path normalization)

[Copilot]

Pull request overview

This PR updates muxt to support specifying --use-templates-variable multiple times so generation, checking, and template-reference inspection can operate across multiple independent *template.Template sets (namespaces), while still detecting route-pattern collisions at generation time.

Changes:

• Make --use-templates-variable variadic across CLI commands, with backward-compatible handling of the deprecated --templates-variable flag.
• Tag each parsed route definition with its originating templates variable and generate handlers that call ExecuteTemplate on the correct variable.
• Add/extend tests + reference txtar fixtures and update CLI/docs to describe multi-variable usage and duplicate route detection.

Reviewed changes

Copilot reviewed 21 out of 22 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
internal/muxt/definition.go	Add templates-variable context to Definitions, normalize patterns, and introduce CheckForDuplicatePatterns.
internal/generate/routes.go	Load/merge definitions from multiple template variables; generate handlers using def.TemplatesVariable().
internal/cli/commands.go	Change --use-templates-variable to slice; add validation/compat helpers; iterate per templates-variable for analysis-style commands.
internal/analysis/check.go	Run template checking per templates-variable.
internal/analysis/routes.go	Make routes output support multiple template variables.
internal/asteval/package.go	Load packages with NeedImports (supports cross-package scenarios).
internal/muxt/definition_test.go	Add tests for duplicate pattern detection and normalization behavior.
internal/cli/commands_test.go	Add tests for templates-variable slice normalization and duplicate detection.
docs/reference/templates-variable.md	Document multiple template variables and behavior implications.
docs/reference/cli.md	Update CLI reference to reflect --use-templates-variable as string[].
cmd/muxt/testdata/*	Add/update script fixtures covering multi-variable generation/checking and error cases.
.gitignore	Ignore .claude/plans.

[crhntr]

Addressed all five Copilot comments in the latest force-push (2692adb):

1. callHandleFunc registration — now uses def.Pattern() (normalized) for mux.HandleFunc and the PathPrefix slicing, so leading/extra whitespace in template names can't produce invalid http.ServeMux patterns at runtime. RawPattern() still feeds debug/error logging.
2. list-template-callers JSON output — accumulates *TemplateCallers results across variables and writes once at the end, so --format=json produces a single valid JSON document.
3. list-template-calls JSON output — same fix.
4. fixTemplateVariables empty value — explicitly rejects --use-templates-variable= with --use-templates-variable value must not be empty instead of letting it fall through to a confusing variable not found. New unit test in internal/cli/commands_test.go covers it.
5. Per-command docs — docs/reference/commands/{check,generate,list-template-calls,list-template-callers}.md updated to string[] with multi-flag wording, matching docs/reference/cli.md.

Also added a JSON-format assertion to reference_list_template_callers_with_multiple_variables.txt so the single-document invariant is regression-tested.