perf(build): use line-tables-only debug info in the dev profile

[Dandandan]

Which issue does this PR close?

Adresses: #13814

Found while profiling compile times for #24325 / #24326 / #24329 / #24330,
which removed the trait-solving cost from the four crates on the critical path
and left LLVM as the dominant remaining cost.

Rationale for this change

dev is the profile behind every cargo build and cargo test, so its debug
info is generated over and over. debug = "line-tables-only" keeps file and line
numbers — panics and RUST_BACKTRACE output stay just as useful — and drops the
variable-level DWARF that only an interactive debugger consumes.

Measured per crate, interleaved with the baseline so machine drift cancels
out. The flag is passed to the crate under test only, so cached dependency
artifacts stay valid and nothing else moves between the two measurements:

crate	debug = 2	line-tables-only
datafusion-physical-plan	8.89s	7.43s	−16%
datafusion-functions-aggregate	5.86s	4.63s	−21%
datafusion-physical-expr	4.57s	3.59s	−21%
datafusion-functions	4.64s	4.04s	−13%
datafusion-expr	4.54s	3.57s	−21%
datafusion-functions-nested	4.37s	3.06s	−30%
datafusion-optimizer	3.91s	3.12s	−20%
datafusion-common	3.73s	3.10s	−17%
datafusion-sql	3.57s	2.43s	−32%
datafusion-datasource-parquet	3.27s	2.44s	−25%
datafusion-datasource	1.90s	1.42s	−25%
datafusion-physical-optimizer	1.22s	0.99s	−19%
sum	50.5s	39.8s	−21%

The saving is codegen-side, as you would expect: datafusion-catalog, which
spends its time in the trait solver rather than in LLVM, moves only 7.4s → 7.0s.

Artifacts shrink as well — libdatafusion_physical_plan.rlib goes from 141MB
to 100MB.

What changes are included in this PR?

One setting on [profile.dev], plus an update to the profile documentation block
above it, which currently advertises "full debug info" for dev.

Are these changes tested?

Are there any user-facing changes?

For anyone stepping through DataFusion in a debugger, local variable inspection
needs CARGO_PROFILE_DEV_DEBUG=2 cargo build (or a local override in
.cargo/config.toml); the comment in Cargo.toml says so. Everything else —
panic locations, backtraces, #[test] failures — is unchanged.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.14%. Comparing base (ab12f5e) to head (50f75f1).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #24339   +/-   ##
=======================================
  Coverage   81.14%   81.14%           
=======================================
  Files        1112     1112           
  Lines      386933   386933           
  Branches   386933   386933           
=======================================
+ Hits       313967   313976    +9     
+ Misses      54476    54463   -13     
- Partials    18490    18494    +4     

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[comphead]

Thanks @Dandandan makes sense for local builds.

[alamb]

🚀