Implement conditional formatting

[ddimaria]

Add conditional formatting support for XLSX

Adds read support for conditional formatting rules from XLSX worksheets. This parses <conditionalFormatting> elements and their child <cfRule> nodes, resolving differential format styles (dxf) into the existing Style type.

Branched from #653, so this should be reviewed after 653 is merged into main.

Supported rule types

Rule type	Variant
Cell value comparison	CellIs (all operators: equal, between, greaterThan, etc.)
Color scales	ColorScale2, ColorScale3
Data bars	DataBar (with fill and border colors)
Icon sets	IconSet (all 20 built-in icon set types)
Top/Bottom N	Top10 (rank, percent, bottom)
Above/below average	AboveAverage (with std deviation support)
Text rules	Text (contains, notContains, beginsWith, endsWith)
Time period	TimePeriod (yesterday, today, last7Days, etc.)
Formula-based	Expression
Blanks/errors	ContainsBlanks, NotContainsBlanks, ContainsErrors, NotContainsErrors
Duplicates/uniques	DuplicateValues, UniqueValues
Fallback	Unknown (preserves raw type string)

API

Two new public methods on Xlsx<RS>:

• worksheet_conditional_formatting(&mut self, name: &str) -- by sheet name
• worksheet_conditional_formatting_at(&mut self, index: usize) -- by sheet index

Both return Vec<ConditionalFormatting>, where each block contains the target sqref range and a list of prioritized rules with resolved Style formats.

Changes

• src/conditional_format.rs (new): Public data model -- ConditionalFormatting, ConditionalFormatRule, ConditionalFormatRuleType, and supporting enums (CfOperator, CfTextOperator, CfValueObject, IconSetType, TimePeriodType, etc.)
• src/xlsx/mod.rs: Parses <dxfs> from styles.xml into differential format styles; adds parse_conditional_formattings and helpers (parse_cf_rules, parse_single_cf_rule, parse_color_scale, parse_data_bar, parse_icon_set, parse_cfvo) to walk the worksheet XML
• src/lib.rs: Re-exports all new public types
• tests/test.rs: 29 new tests covering every rule type variant, multi-rule blocks, stop_if_true, reversed icon sets, bottom-percent, std-dev below-average, unknown types, and dxf style resolution
• tests/conditional_formatting.xlsx (new): Test fixture with 25 conditional formatting blocks

Example & Benchmarks

• examples/conditional_formatting.rs: Demonstrates reading and printing all conditional formatting rules from a workbook
• benches/conditional_formatting.rs: Criterion benchmarks for three scenarios:
  • cf_small_25_blocks -- real-world file with 25 CF blocks
  • cf_large_200_blocks_1000_rules -- synthetic file with 200 blocks / 1,000 rules
  • cf_large_with_dxf_access -- large file with dxf style resolution

Run with:

cargo bench --bench conditional_formatting

Test plan

• All 29 new conditional formatting tests pass
• Existing test suite unaffected
• Example runs against test fixture: cargo run --example conditional_formatting
• Benchmarks compile and run: cargo bench --bench conditional_formatting

[jmcnamara]

PR #538 will have to be resolved before this can be reviewed/merged.

[jmcnamara]

@ddimaria just to give you a heads up that I will get to this and merge it in the next release +1. Can you keep it up to date with the current main branch and resolve any conflicts.