Merge RFC-001: Expression DSL Extension

[Khanathan]

RFC-001: Expression DSL Extension

Linked issues

Related to #56

Summary

Adds rfcs/RFC-001-Expressions.md — an RFC that proposes
letting users write streaming transforms in plain Python and
expands the expression DSL with 11 new builtins. No code changes,
RFC only.

What it proposes

• @bv.expr decorator. Write transforms with normal Python:
  if/elif/else, ternaries, and/or/not, is None,
  local variables. The decorator translates them into the
  existing DSL.
• 11 new v0 builtins across 5 categories:
  • Math: log1p, clip
  • String: lower, contains, starts_with, ends_with, replace
  • Conditional / null: if_else, is_in
  • Time: hour_of_day
  • Hashing: hash_mod
• Event dot-access. e.email as sugar for bv.col("email").
• Generalized typechecking. Move per-builtin type rules from
  a hardcoded match fn_name in infer_call_type onto an
  infer fn pointer on each BuiltinFn row. Compiler enforces
  coverage.
• CONTRIBUTING-OPS.md walking a full op contribution so
  future builtins are good-first-issues.

What it does not change

• No new Expr / Literal variants. No grammar growth. No
  wire-format changes.

Why now

Today's DSL stops at arithmetic, comparisons, and three builtins
(cast, isnull, quadkey). Users hit the gap on basic shapes
(log/clip, lower/contains, hour-of-day, conditional branching)
and route around it with offline Python — losing the windowed
aggregations the server exists to provide. This RFC closes the
gap with the smallest possible surface change and makes future
builtins a one-row append.