Report handler panics through the logger and answer 500 (closes #187)

chi v1.5.5's middleware.Recoverer neither logged a handler panic nor
answered 500. Its pretty-printer scans the stack for a frame beginning
"panic(0x", which the runtime no longer emits, so the scan never
terminates early and every line reaches decorateFuncCallLine, which
slices pkg[strings.Index(pkg, "."):] without checking for -1. That
second panic escaped chi's own deferred function, so its
WriteHeader(500) never ran: net/http closed the connection and reported
its own crash, losing the original panic value entirely.

Middleware.Recoverer replaces it. It writes one ERROR record through
internal/logger carrying the panic value, the stack and the request id,
and answers 500. http.ErrAbortHandler is re-panicked rather than
swallowed, and a response the handler already committed is left alone
rather than overwritten.

It is registered inside every middleware that observes the response, so
the 500 is the status the access log records and the metrics count, and
outside the sentryhttp handler, whose Repanic option needs something
further out to catch what it re-raises.

Both fields are bounded in encoded bytes, through the same
internal/logfield budget the access log spends: 512 for the panic value,
since a handler may build one out of the request, and 8192 for the
stack, cut at its far end so the panic site survives.
MaxPanicLogLineBytes states the resulting ceiling at 10240; measured,
the widest line either handler produces is 8898, a figure that carries
no source paths and reproduces across checkouts. The real case through
the shipped chain measures roughly 3960 bytes; that one moves with the
checkout, because debug.Stack() embeds absolute source paths, so it is
stated as a measurement rather than as an invariant and no test asserts
it.

Because the panic record no longer reaches net/http's error log, the
carve-outs in README.md and in the MaxAccessLogLineBytes doc comment
that described that path are removed rather than reworded. What
replaces them states the ceiling the record is now written under, and
internal/server/recoverer_test.go asserts that "http: panic serving"
appears in neither of the process's streams.

internal/middleware/middleware.go

@@ -133,16 +133,12 @@ const (
 	//   - The "log" delivery target, which exists to write the whole
 	//     inbound event to the log. Deliberate; see
 	//     internal/delivery/target_log.go.
-	//   - The widest line the service can write, which is neither an
-	//     access log line nor client-chosen. A handler panic arrives
-	//     through net/http's nil ErrorLog as one record carrying a
-	//     whole goroutine stack, above this figure — measured at
-	//     roughly 2,770 bytes. The exact width is not an invariant: it
-	//     moves with the goroutine number and with the source paths
-	//     baked into the stack. That it exceeds this ceiling does not
-	//     move. The value is the runtime's, not a client's; see
-	//     README.md and
-	//     https://git.eeqj.de/sneak/webhooker/issues/187.
+	//   - The record a recovered panic writes, which is neither an
+	//     access log line nor client-chosen: it carries a whole
+	//     goroutine stack and is wider than this figure. It has its
+	//     own stated ceiling, MaxPanicLogLineBytes in recoverer.go,
+	//     and is written once per recovered panic rather than once
+	//     per request.
 	MaxAccessLogLineBytes = 2560
 )