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enable "thin LTO" in new `dist` profile (#23527)

This is a followup and replacement for
https://github.com/pantsbuild/pants/pull/23179 following the review
feedback there. Instead of adding more optimizations to the `release`
profile it creates a new `dist` profile. The bikeshed naming logic for
the profile breakdown:
* `release`: This is the built in cargo name so we are stuck with it. We
use it by default and it does "some" optimization which is a good
balance for the (I think) most common case of someone working on Python
rules code.
* `dist`: Intended to have maximum optimization for release artifacts.
It is same name used by `ruff` and `uv`. I moved the `codegen-units`
here as requested before.
 * `debug`: Whatever cargo defaults to.

In various tests with hyperfine for the prior PR I saw like `1.05 ±
0.06`, or `1.02 ± 0.09`. So not going to write a blog post about it, but
we don't seem to hit any pathological corner cases and I'll take a few
percentage points for free.

Rough Summary Table:

| Cold Build                  | Wall Time | `libengine.so` |
|-----------------------------|-----------|----------------|
| Old release (cgu=1, no LTO) | 2m33s     | 189.9 MiB      |
| New release (cgu=16)        | 1m53s     | 247.1 MiB      |
| dist (thin LTO, cgu=1)      | 3m57s     | 173.8 MiB      |

References:
*
https://nnethercote.github.io/perf-book/build-configuration.html#link-time-optimization
 * https://doc.rust-lang.org/cargo/reference/profiles.html#lto
 
NOTICE: Futzing with bash done my an LLM.

closes #23179

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