Apply PR #110's Read index doc comment fix to the v2 legacy index module

[sinmetal]

Summary

v2/module/builtin/templates/legacy_index.go.tpl hardcodes the doc comment on every generated Read{{ .LegacyFuncName }} as // Generated from unique index '...'., even when the underlying index is not unique. Users of the legacy index module (--use-legacy-index-module) therefore still see misleading doc comments on non-unique-index ReadXxx functions.

This is the same issue that #110 (merged 2022-12-07) fixed for module/builtin/templates/index.go.tpl, and that #205 backported to the v1 templates. legacy_index.go.tpl was missed by both, so after #205 the v1 output and the v2 legacy output disagree on the same comment:

// v1 (after #205)                      // v2 legacy (today)
// Generated from index '...'.          // Generated from unique index '...'.
func ReadCompositePrimaryKeysByError(   func ReadCompositePrimaryKeysByError(

Change

Mirror #110 and #205 exactly — drop the word unique from the Read{{ .LegacyFuncName }} doc comment so it reads // Generated from index '...'. regardless of uniqueness:

+// Generated from index '{{ .IndexName }}'.                                                                              func Read{{ .LegacyFuncName }}(ctx context.Cont.KeySet) ([]*{{ .Type.Name }}, error) {

v2/test/testmodels/legacy_default/**/*.yo.go is regenerated via make -C v2 testdata-from-ddl to reflect the template change.

The Find{{ .LegacyFuncName }} comment is left nside an {{ if .IsUnique }} branch, so uniqueindex is accurate there.

Scope

• Comment-only change. No runtime behavior change.
• Touches only v2/module/builtin/templates/legaegenerated v2/test/testmodels/legacy_default/`(5 files, 17 lines).
• v2/test/testmodels/default/ and `dump_types/legacy index module was already fixed by Fix typo in index.go.tpl #110.
• v2 embeds templates with go:embed (v2/module/builtin/module.go), so there is no v1-style tplbin regeneration step.
• The v1 tree is untouched (already fixed by use handy-spanner for tests #20