fib, bench: profile the daemon taking in a full table; count blackholes instead of scanning

Before this change there was no way to see where the daemon's CPU time
goes while it receives a full Internet routing table: the benchmarks in
crates/bench time the routing table in-process, without the sessions,
the policy, the kernel task or the production allocator. Profiling the
daemon itself showed that a third of its CPU time during a full-table
load went into `Plan::blackholes()`, which walked every installed route
to count the blackhole ones each time the kernel task published its
status, after every batch of changes; with 1.2 million routes
installed that was 3.9 seconds from the first route to a synced kernel
instead of 2.0.

This change:

* Keeps counts of blackhole routes and of routes kept unconfirmed from
  an earlier run in `Plan`, in step with the map through `Plan::put`
  and `Plan::take`, so that `Plan::blackholes()` and `Plan::kept()`
  cost nothing. `kept()` had the same scan; it ran only while routes of
  an earlier run were being kept, which is exactly during a full-table
  load after a restart.
* Adds `scripts/profile/full-table.sh`, which builds the daemon with the
  new `profiling` Cargo profile (release with line tables) and frame
  pointers, runs it in a network namespace joined by a veth pair to a
  second namespace, feeds it the synthetic table from there as one or
  more neighbors, samples it with `perf`, and writes a flame graph
  (inferno), flat profiles by self and inclusive time, the time until
  the routes are in the table and in the kernel, and the daemon's
  resident memory. `PEERS`, `KERNEL`, `SCALE_DOWN`, `HOLD_SECS`, `FREQ`
  and `OUT` control it. From macOS it runs in the Linux VM through
  scripts/linux-vm.sh.
* Adds the load generator `crates/bench/examples/feed.rs`: it connects
  from the peers' addresses, completes the handshakes, writes each
  neighbor's table, drains what the daemon sends back and holds the
  sessions open with KEEPALIVEs. `TestPeer::into_stream` gives it the
  connection after the handshake.
* Updates the testing guide: the current `full_table` numbers, a
  "Profiling" section, and the daemon's numbers in the VM.

Measured in the Linux VM (8 vCPUs) with the synthetic table of 1.23
million prefixes: one neighbor's table is in the routing table after
0.7 s and in the kernel after 2.0 s (before: 3.9 s); four neighbors'
4.9 million routes are in the table after 1.1 s and the kernel is
synced after 3.0 s; 520 MB resident with one table, 680 MB with four.
Without the kernel one session takes in 2.0 million routes per second
on one core. A profile also attributed 14% of the session's time to
`Journal::tidy`; inlining its early return changed nothing in an A/B
run of the benchmark, so that was a sampling artefact and is left as it
is.

Testing: a unit test checks the counts against a scan of the entries
through adopt, best, reset, synced, release, withdrawal, refusal and
clear; the kernel test suite and the workspace tests pass; the script
was run with one and four neighbors, with and without the kernel.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Cargo.toml

@@ -65,3 +65,10 @@ tokio-stream = { version = "0.1.19", features = ["sync"] }
 reqwest = { version = "0.13.5", default-features = false, features = ["json"] }
 # The daemon's memory allocator; see bin/bgp/src/main.rs.
 tikv-jemallocator = "0.7.0"
+
+# `cargo build --profile profiling`: the release build, with the line tables
+# a profiler needs to name functions. See scripts/profile/.
+[profile.profiling]
+inherits = "release"
+debug = "line-tables-only"
+strip = "none"