feat: add FormState for re-rendering forms after validation

[acoshift]

Summary

Tier-2 website-DX: ctx.FormState() returns a *FormState that holds the submitted form values together with per-field validation errors, so a form can be re-rendered with the user's input after a failed submission (instead of making them retype).

func signup(ctx *hime.Context) error {
    form := ctx.FormState()
    if !strings.Contains(form.Value("email"), "@") {
        form.AddError("email", "Enter a valid email")
    }
    if form.HasErrors() {
        return ctx.View("signup", map[string]any{"Form": form})
    }
    // ... create the account ...
    return ctx.Redirect("/welcome")
}

<input name="email" value="{{.Form.Value "email"}}">
{{if .Form.HasError "email"}}<span class="err">{{.Form.Error "email"}}</span>{{end}}

API

• Values: Value(name), Values(name), SetValue(name, value) (pre-fill, e.g. an edit form)
• Errors: AddError(name, msg), Error(name) (first), Errors(name) (all), HasError(name), HasErrors()

Design notes (open to redirection)

• Handler-passes-to-view, no per-request template funcs — keeps it self-contained (unlike isRoute, which still needs that enabler).
• Values are copied from the parsed form, so SetValue never mutates ctx.Request.Form (tested).
• Seeded from query and body (matching ctx.FormValue); uses ParseMultipartForm, so it works for urlencoded and multipart text fields.
• Categorized, multi-error (map[string][]string) — a field can carry several messages.

Test plan

• go vet ./...
• go test ./... — 100% coverage on all 10 FormState functions (incl. the no-mutation guarantee)
• go test -race ./...
• gofmt clean
• runnable ExampleContext_FormState; CLAUDE.md updated

Tier-2 status

This leaves isRoute as the last Tier-2 item — gated on the per-request template-funcs enabler decision.

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