Fix RBI NACH scraping to capture bank_code and bank_name correctly

[PriyankaMarbill]

RBI added a Bank Name column (index 2) to the NACH participation table, shifting column indices for MICR/IFSC/IIN/ACH onwards. The scraper was reading the wrong columns for bank_code and silently dropping bank_name.

Also, RBI periodically renames columns in the RTGS/NEFT CSV exports (e.g. "IFSC Code" instead of "IFSC", "Bank Name" instead of "BANK"). The CSV parser now normalises column names via an alias map so column renames no longer silently drop data.

Changes:

• methods_nach.rb: capture bank_name from data[2] (new RBI column) and pass it through bank_data; parse_nach extracts it per row
• methods.rb: add RBI_HEADER_ALIASES + helpers to normalise CSV column names; update parse_csv to build a header map on first data row; add normalize_bank_name_for_guidelines to apply CONTRIBUTING.md rules mechanically; add export_banknames to emit data/banknames.json with the most-frequent RBI name per bank code merged with existing src/banknames.json; update merge_dataset to prefer the RBI-supplied bank name over the static lookup; update parse_imps to fall back to NACH bank_name for codes not yet in banknames.json; fix IFSC skip check to also handle "IFSC Code" header rows mid-sheet
• generate.rb: call export_banknames after dataset is built
• CONTRIBUTING.md: note that the scraper auto-applies name guidelines

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