docs: bring EBNF grammar and project docs up to date

- grammar.ebnf: rewrite the number rule to cover the literal syntaxes the reader
  now accepts — 0x/0X hex, N (bigint) / M (bigdec) suffixes, ratios a/b, radixed
  integers (NrXXX, base 2..36), exponents, and the ##Inf/##-Inf/##NaN symbolic
  floats — noting Jolt reads them as plain Janet numbers.
- README: Numbers bullet notes the literal syntaxes read; conformance count.
- reader-syntax-spec: drop the stale 'ratio not supported' case; add coverage
  for hex-uppercase/N/M/ratio/radix/exponent/##Inf/##NaN.
- PLAN.md: refresh the stale Current State snapshot for the 3-layer test
  structure (spec/integration/unit), ~3,920 suite assertions, 218/218
  conformance, current source size.

jpm test green.

test/spec/reader-syntax-spec.janet

@@ -12,8 +12,15 @@
   ["namespaced keyword" "true"      "(= :a/b :a/b)"]
   ["char"               "\\a"       "\\a"]
   ["char newline"       "true"      "(= \\newline (first \"\\n\"))"]
-  ["ratio not supported but reads ints" "3" "3"]
   ["hex literal"        "255"       "0xff"]
+  ["hex uppercase"      "31"        "0X1F"]
+  ["bigint suffix N"    "42"        "42N"]
+  ["bigdec suffix M"    "1.5"       "1.5M"]
+  ["ratio -> double"    "0.75"      "3/4"]
+  ["radix integer"     "255"        "16rFF"]
+  ["exponent"           "1500.0"    "1.5e3"]
+  ["symbolic Infinity"  "true"      "(infinite? ##Inf)"]
+  ["symbolic NaN"       "true"      "(NaN? ##NaN)"]
   ["symbol via quote"   "'foo"       "'foo"])
 
 (defspec "reader / collection literals"