fix: detect ATX headings indented up to 3 spaces

[zerochae]

Problem

The Markdown parser missed ATX headings that had any leading whitespace.
# Title was detected, but # Title (indented) was not — even though
CommonMark treats up to 3 leading spaces as a valid heading.

Cause

The heading pattern was anchored at column 0:

local hashes, rest = line:match "^(#+)%s+(.*)$"

Any space before # made the match fail.

Fix

Allow 0–3 leading spaces, and keep 4+ spaces as an indented code block
(per CommonMark):

local indent, hashes, rest = line:match "^( *)(#+)%s+(.*)$"
if hashes and #hashes <= 6 and #indent <= 3 then

Tests

Added regression tests in tests/parser_spec.lua:

• headings with 1–3 leading spaces are detected ( # H2, ### H3)
• a heading indented 4+ spaces is ignored (indented code block)

Full suite: 78 passed / 0 failed.

Scope

Other parsers were audited for the same class of bug. Org headlines, reST
section adornments, and Vim-help rules are all required to sit in column 0 by
their formats, so their column-0 anchoring is correct — no change needed. The
HTML heading matcher already allows leading whitespace.