feat: add number formatting options for display (#197)

[wellorbetter]

Description

Add number formatting options for display rendering.

Closes #197

Reasoning

CSV files often contain raw numeric data that is hard to read at a glance (e.g. 23.505744680851063, 1234567, 2500000000).
This change adds display-layer formatting so numbers can be rendered in a more human-readable form without affecting the
underlying data — search, sort, and copy all continue to use original values.

Four formats are supported:

Format	Flag	Example input	Example output
thousands	--number-format thousands	1234567.89	1,234,567.89
fixed	--number-format fixed	23.505744680851063	23.51
si	--number-format si	2500000000	2.50B
scientific	--number-format scientific	0.00123	1.23e-3

Non-numeric values (dates, strings, empty cells) are passed through unchanged.

An optional --precision N parameter controls decimal places for fixed, si, and scientific formats (default: 2).

Usage

# Apply one format to all columns
csvlens data.csv --number-format fixed --precision 2

# Per-column format
csvlens data.csv --column-format "revenue=thousands" --column-format "rate=fixed" --precision 3

# SI units (useful for large counts)
csvlens data.csv --number-format si

# Precision alone has no effect (warns the user)
csvlens data.csv --precision 2
# Warning: --precision has no effect without --number-format or --column-format

Testing

1. Run cargo test — 40 new unit tests in src/format.rs, all passing
2. Open examples/test_format.csv with each format and verify display:

   target/release/csvlens examples/test_format.csv --number-format fixed --precision 2
   target/release/csvlens examples/test_format.csv --number-format thousands
   target/release/csvlens examples/test_format.csv --number-format si
   target/release/csvlens examples/test_format.csv --number-format scientific --precision 2
   target/release/csvlens examples/test_format.csv --column-format "value=fixed" --column-format "count=thousands" --precision 3

3. Confirm search/filter on formatted columns still uses original values
4. Confirm non-numeric columns (e.g. date) render unchanged
5. Confirm --no-headers + --column-format prints a warning

Type of change

• Bug fix (A non-breaking change that fixes an issue)
• New feature (A non-breaking change that adds functionality)
• Breaking change (A fix or feature that would cause existing functionality to not work as expected)
• Refactor (A code change that neither fixes a bug nor adds a feature)
• Performance (A code change that improves performance)
• Style (Code style changes)
• Docs (Changes to documentation)
• Chore (Changes to the build process or other tooling)

[wellorbetter]

Implements number formatting for display rendering (issue #197).

New module src/format.rs provides four formats — thousands, fixed, si, scientific —with an optional --precision N parameter. Formatting is applied at the render layer only
(src/ui.rs), so search/sort/copy continue to use original values.

Changes:

• src/format.rs: core formatting logic + 40 unit tests
• src/runner.rs: three new CLI args (--number-format, --column-format, --precision)
• src/app.rs: pass config as Arc<ColumnFormatConfig> to render state each frame
• src/ui.rs: apply format in render_row and get_column_widths for correct column sizing
• src/lib.rs: expose pub mod format