chore: disable incremental compilation and trim debug info for dev builds

[vsbuffalo]

Cargo's dev defaults (incremental = true, debug = 2) are a good trade in
the normal regime: more files on disk for faster edit-compile cycles.
This workspace crossed a scale threshold where that stops holding on
macOS.

target/debug had reached 88 GB across ~1.67M files — 44 GB of
incremental/ and 44 GB of deps/ — against a 4.1 GB target/release.
Every Mach-O artifact in there is assessed by syspolicyd (Gatekeeper),
which sat at ~70% of one core and had accumulated 192 hours of CPU over
26 days of uptime. A debug cargo test took 6m52s at 4% CPU: not
compute-bound, but waiting on filesystem and security-assessment work
generated by the tree itself.

Deleting target/debug is what recovers the build time; this commit is
what stops it coming back. Measured cold on identical source, after that
deletion:

                    stock dev      this profile
wall                27.2s          21.2s
CPU work            176s           126s
target/debug        2.5 GB         1.1 GB
files               18,647         3,181
incremental cache   1.1 GB         0 B

The file count is the load-bearing number, since that is what is
assessed. 5.9x fewer artifacts per build, and no incremental cache to
accumulate — 1.1 GB of it regenerated from a single cold build, which is
the rate that produced 44 GB over 26 days of continuous agent builds.

line-tables-only for workspace crates and no debug info for
dependencies is Cargo's own build-performance recommendation: panic
backtraces still resolve to source locations without paying for full
DWARF. A debugging profile inheriting from dev keeps debug = 2
available for the rare case that wants a real debugger, rather than
making every CI and agent build carry metadata it does not use.

split-debuginfo is deliberately NOT changed. macOS defaults dev builds
to unpacked, packed runs dsymutil on every build, and with debug
info cut this far there is little left to pack. It wants its own
benchmark before anyone touches it.

This encodes the fix in the repo rather than relying on everyone
remembering CARGO_INCREMENTAL=0.