htmx 4: update http package headers

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Part of #18. Reference: https://four.htmx.org/reference/headers and the header tables in https://four.htmx.org/docs/whats-new-in-htmx-4

## Request headers

- Remove `GetTrigger` (`HX-Trigger` request header replaced by `HX-Source`).
- Remove `GetTriggerName` (folded into `HX-Source`).
- Remove `GetPrompt` (extension only).
- Add `GetSource` for `HX-Source`: the triggering element as `tag#id`, e.g. `button#submit`.
- Add `GetRequestType` for `HX-Request-Type`: `"full"` or `"partial"`.
- `GetTarget` value format changed from bare id to `tag#id` (or just the tag when there is no id). Update the doc comment; no parsing helper.

## Response headers

- Remove `SetTriggerAfterSettle` and `SetTriggerAfterSwap` (fold into `HX-Trigger`).
- Add `SetReselect` for `HX-Reselect` (exists in htmx 2 as well, never added here).
- `SetTrigger` doc: events now fire after the swap completes and support a `target` key.

Unchanged: `IsRequest`, `IsBoosted`, `GetCurrentURL`, `IsHistoryRestoreRequest`, `SetLocation`, `SetPushURL`, `SetRedirect`, `SetRefresh`, `SetReplaceURL`, `SetReswap`, `SetRetarget`.

## Acceptance criteria

- The table-driven tests in `http/http_test.go` cover exactly the htmx 4 header set.
- Package doc links to https://four.htmx.org/reference/headers

Start with the existing helpers in the http package and the table-driven tests in http/http_test.go, then compare them with the htmx 4 header reference. Done means the tests cover exactly the requested htmx 4 header set, including additions, removals, and format changes, and the package documentation links to the htmx 4 reference.

Written by the indexing model from the issue text.