feat: add multipart call-site identifier

[crhntr]

Summary

• Adds a new multipart call-site identifier to muxt template names, peer to the existing form identifier, that triggers request.ParseMultipartForm and supports binding *multipart.FileHeader / []*multipart.FileHeader struct fields alongside all existing form field types.
• Two modes (mirroring form): raw mode (param type *multipart.Form — the parsed form is handed over directly) and struct-binding mode (text fields parsed via the existing generateParseValueFromStringStatements codepath; file fields bound from request.MultipartForm.File).
• New CLI flag --output-multipart-max-memory=<size> accepts human-readable byte sizes (32MB, 64MiB, 1GB) via github.com/dustin/go-humanize. Default 32 MiB.
• ParseMultipartForm errors are captured into td.errList with td.errStatusCode = http.StatusBadRequest (diverges from the silent-ignore behavior of ParseForm because multipart parse errors are real client-side faults). File reads are guarded by if request.MultipartForm != nil since the stdlib leaves it nil on parse failure.
• Mutual exclusion: form and multipart in the same call site is a generation error.
• Reuses existing name and template struct tags — no new tag namespace.
• Documentation updated: docs/reference/template-names.md parameter-source table.

Test plan

• go test ./... passes
• 10 new txtar cases in cmd/muxt/testdata/:
  • reference_multipart_basic, _multiple_files, _mixed, _raw, _with_name_tag
  • reference_multipart_max_memory_flag — asserts the literal ParseMultipartForm(<bytes>) and a functional upload test
  • reference_multipart_parse_error — functional test: malformed multipart body → 400 via td.errList
  • err_multipart_with_form, err_multipart_unsupported_field_type
  • howto_multipart_file_upload — task-oriented walkthrough
• gofumpt clean

Out of scope

• Numeric-suffix identifiers like multipart32MB for per-route limits
• Per-field tags for max file size or content-type allowlist (maxsize, accept)
• Streaming uploads via *multipart.Reader
• Auto-detecting multipart from Content-Type at runtime
• Changing ParseForm error-handling semantics for the existing form identifier

Note: humanize uses standards-correct unit semantics — KB/MB/GB are decimal (1000-based), KiB/MiB/GiB are binary (1024-based). Use MiB if you want power-of-two values.

Assisted-by: Claude:claude-opus-4-7 gopls staticcheck gofumpt

[Copilot]

Pull request overview

Adds first-class multipart form handling to muxt template call-site identifiers, enabling generated handlers to parse multipart/form-data requests and bind uploaded files into strongly-typed receiver method parameters.

Changes:

• Introduces a new multipart template call-site identifier (mutually exclusive with form) and adds generation support for raw *multipart.Form and struct-binding modes.
• Adds --output-multipart-max-memory=<size> CLI flag (human-readable bytes) to control request.ParseMultipartForm’s maxMemory.
• Updates docs and expands txtar-based generator tests for multipart parsing, binding, and error cases.

Reviewed changes

Copilot reviewed 15 out of 16 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
internal/muxt/definition.go	Extends the recognized template call-site scope identifiers to include multipart.
internal/generate/routes.go	Implements multipart parsing + struct binding (including *multipart.FileHeader and []*multipart.FileHeader) and enforces form/multipart mutual exclusion.
internal/cli/commands.go	Adds CLI flag parsing for multipart max memory via go-humanize.
go.mod	Adds github.com/dustin/go-humanize dependency.
go.sum	Updates sums for the new dependency and dependency graph changes.
docs/reference/template-names.md	Documents multipart identifier semantics, supported types, mutual exclusion, and maxMemory flag.
cmd/muxt/testdata/reference_multipart_basic.txt	Adds generator + runtime test for single *multipart.FileHeader binding.
cmd/muxt/testdata/reference_multipart_multiple_files.txt	Adds generator + runtime test for []*multipart.FileHeader binding.
cmd/muxt/testdata/reference_multipart_mixed.txt	Adds generator + runtime test for mixed text + file multipart struct binding.
cmd/muxt/testdata/reference_multipart_raw.txt	Adds generator + runtime test for raw *multipart.Form parameter mode.
cmd/muxt/testdata/reference_multipart_with_name_tag.txt	Adds generator + runtime test for name tag rebinding with multipart fields.
cmd/muxt/testdata/reference_multipart_max_memory_flag.txt	Adds test asserting ParseMultipartForm(<bytes>) literal respects the new flag.
cmd/muxt/testdata/reference_multipart_parse_error.txt	Adds runtime test asserting malformed multipart bodies produce 400 via td.errStatusCode.
cmd/muxt/testdata/err_multipart_with_form.txt	Adds negative test ensuring form and multipart are rejected together.
cmd/muxt/testdata/err_multipart_unsupported_field_type.txt	Adds negative test for unsupported multipart-bound field types.
cmd/muxt/testdata/howto_multipart_file_upload.txt	Adds a task-oriented walkthrough test case for multipart file uploads.