Support EDN symbolic values (##Inf/##-Inf/##NaN) so non-finite floats round-trip

[gaoflow]

Problem

dumps() silently corrupts non-finite floats. inf, -inf, and nan are emitted as the bare tokens inf / -inf / nan, which the reader then reads back as Symbol objects rather than floats:

>>> import edn_format as e
>>> e.loads(e.dumps(float("inf")))
Symbol(inf)          # expected: inf
>>> e.loads("##Inf")
EDNDecodeError: Illegal character '#' ...

So loads(dumps(x)) does not round-trip for non-finite floats (the repo's own check_roundtrip invariant), and the canonical EDN representation produced by Clojure (##Inf / ##-Inf / ##NaN) cannot be read at all.

Fix

Support EDN's symbolic values, ##Inf / ##-Inf / ##NaN, on both paths:

• edn_lex.py: a SYMBOLIC_VALUE token (\#\#(-Inf|Inf|NaN)), defined ahead of the other # token rules so it takes lexer priority. It maps to the matching float.
• edn_parse.py: SYMBOLIC_VALUE added to the p_term_leaf rule.
• edn_dump.py: non-finite floats now dump to ##Inf / ##-Inf / ##NaN instead of bare tokens.

Non-finite floats now round-trip, and ##Inf / ##-Inf / ##NaN from real EDN parse correctly. The non-finite check uses math.isnan / math.isinf (available on Python 2.6+) to stay within the package's stated version support.

Tests

Added test_symbolic_values to EdnTest, covering both parse and dump directions plus the existing check_roundtrip helper. It fails before the change (EDNDecodeError) and passes after. Full suite: 39 tests pass; flake8 --max-line-length=100 clean. Existing #-dispatch syntax (#{...}, #_, #inst, #uuid, #:ns{...}) is unaffected.

I used an AI assistant to help investigate this under my direction, and I have reviewed and verified the change myself.

[bfontaine]

Merged; thank you!