server: DF-mode big_send + uploaded-run storage with an operator policy

big_send now forces the Don't-Fragment bit for the whole burst by default, so
the largest size that arrives IS the downstream path MTU rather than "fragments
got through" — two different measurements the schema already separates. Sizes
above our own egress MTU (from the startup self-test) are refused up front and
reported as max_df_bytes, because absence caused by our kernel must not be read
as a limit of the client's path.

Uploads: one JSON file per run under the state dir, with the policy the operator
actually cares about — who may upload (off / anonymous / account), how large,
how long to keep, and the least anonymization accepted. The profile advertises
all of it so the app can present the switch honestly instead of discovering the
rules by failing. `account` refuses today rather than falling back to anonymous:
picking the strict setting before OIDC lands must not silently mean the loose one.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/internal/dataplane/df_other.go

@@ -0,0 +1,16 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build !linux
+
+package dataplane
+
+import "net"
+
+// Forcing DF per-socket is Linux-specific (IP_MTU_DISCOVER). Off Linux the
+// send still happens — just without the guarantee that nothing fragmented it,
+// so the caller must report the result as fragment-delivery evidence rather
+// than a path-MTU measurement. See dfSupported.
+func withDF(conn *net.UDPConn, fn func() error) error { return fn() }
+
+const dfSupported = false